Suggestion of improved version of get_contourf_levels() concerning the contour labels and float formatting

[AndreasLuckert]

As I coulnd't figure out how exactly to start a pull request here (never done that before actually),
I'm going to post my customized version of the function get_contourf_levels().
This will allow for passing a specific float-format of desire which will affect the labeling of the contours.
The formatting of these string-labels itself has also been improved.

def get_contourf_levels(levels, extend, float_fmt="%.2f"):
    # Latest implementation using f-strings and improved formatting of all ranges
    mid_levels = [
        f"({float_fmt % levels[i]} - {float_fmt % levels[i+1]})"
        for i in range(len(levels) - 1)
    ]
    # Previous implementation:
    #mid_levels = [float_fmt % levels[i] + '-' + float_fmt % levels[i+1] for i in range(len(levels)-1)]

    # i) Decide whether to modify the mid-levels list
    if extend == 'both':
        return [
            f"< {float_fmt % levels[0]}", *mid_levels,
            f"> {float_fmt % levels[-1]}"
        ]
    elif extend == 'max':
        return [*mid_levels, f"> {float_fmt % levels[-1]}"]
    elif extend == 'min':
        return [f"< {float_fmt % levels[0]}", *mid_levels]

    # ii) Return unchanged
    else:
        return mid_levels

To pass the new parameter float_fmt, the two contourf-geojson functions also need to be adjusted a bit:

def contourf_to_geojson_overlap(contourf,
                                geojson_filepath=None,
                                min_angle_deg=None,
                                ndigits=5,
                                unit='',
                                stroke_width=1,
                                fill_opacity=.9,
                                geojson_properties=None,
                                strdump=False,
                                serialize=True,
                                float_fmt="%.2f"):

    """Transform matplotlib.contourf to geojson with overlapping filled contours."""
    polygon_features = []
    contourf_idx = 0

    # Pass (custom) float format to extract the contour-levels
    contourf_levels = get_contourf_levels(contourf.levels,
                                          contourf.extend,
                                          float_fmt=float_fmt)

    for collection in contourf.collections:
        color = collection.get_facecolor()
        for path in collection.get_paths():
            for coord in path.to_polygons():
                if min_angle_deg:
                    coord = keep_high_angle(coord, min_angle_deg)
                coord = np.around(coord, ndigits) if ndigits else coord
                polygon = Polygon(coordinates=[coord.tolist()])
                fcolor = rgb2hex(color[0])
                properties = set_contourf_properties(
                    stroke_width, fcolor, fill_opacity,
                    contourf_levels[contourf_idx], unit)
                if geojson_properties:
                    properties.update(geojson_properties)
                feature = Feature(geometry=polygon, properties=properties)
                polygon_features.append(feature)
        contourf_idx += 1
    feature_collection = FeatureCollection(polygon_features)
    return _render_feature_collection(feature_collection, geojson_filepath,
                                      strdump, serialize)

def contourf_to_geojson(contourf,
                        geojson_filepath=None,
                        min_angle_deg=None,
                        ndigits=5,
                        unit='',
                        stroke_width=1,
                        fill_opacity=.9,
                        fill_opacity_range=None,
                        geojson_properties=None,
                        strdump=False,
                        serialize=True,
                        float_fmt="%.2f"):
    """Transform matplotlib.contourf to geojson with MultiPolygons."""

    if fill_opacity_range:
        variable_opacity = True
        min_opacity, max_opacity = fill_opacity_range
        opacity_increment = (max_opacity - min_opacity) / len(contourf.levels)
        fill_opacity = min_opacity
    else:
        variable_opacity = False
    polygon_features = []

    # Pass (custom) float format to extract the contour-levels
    contourf_levels = get_contourf_levels(contourf.levels,
                                          contourf.extend,
                                          float_fmt=float_fmt)

    for coll, level in zip(contourf.collections, contourf_levels):
        color = coll.get_facecolor()
        muli = MP(coll, min_angle_deg, ndigits)
        polygon = muli.mpoly()
        fcolor = rgb2hex(color[0])
        properties = set_contourf_properties(stroke_width, fcolor,
                                             fill_opacity, level, unit)
        if geojson_properties:
            properties.update(geojson_properties)
        feature = Feature(geometry=polygon, properties=properties)
        polygon_features.append(feature)
        if variable_opacity:
            fill_opacity += opacity_increment
    feature_collection = FeatureCollection(polygon_features)
    return _render_feature_collection(feature_collection, geojson_filepath,
                                      strdump, serialize)