Proper callable support in fit_curve and related

[soxofaan]

from openEOPlatform/documentation#10

curve_fitting = l2a_bands.fit_curve(
    parameters=[1,1,1], # Initial guess of the parameters
    dimension="t",      # Fit the function along the temporal dimension
    function=l2a_bands._get_callback(fit_function, parent_parameters=["data","parameters"])
)

it's unfortunate that you have to call a private method l2a_bands._get_callback like that, we should make sure that you can just do function=fit_function directly