[Work in progress] adds custom basemap layer

[andy-esch]

This PR adds custom vector basemap layers to the cartoframes.contrib.vector module.

Basemaps can be CARTO VL styles or Mapbox styles (if access token is provided).

Full example:

from cartoframes.contrib import vector
from cartoframes import CartoContext, Layer
cc = CartoContext(
    base_url='https://eschbacher.carto.com',
    api_key='abc123'
)

layers = [
    vector.Layer('all_month_3'),
]
# external basemap
vector.vmap(
    layers,
    context=cc,
    basemap={
        'style': 'mapbox://styles/mapbox/streets-v9',
        'token': 'def456'
    })

# carto basemap
vector.vmap(
    layers,
    context=cc,
    basemap=vector.BaseMaps.darkmatter
)

[coveralls]

Coverage decreased (-0.5%) to 96.635% when pulling 6cf627c on custom-vector-basemap into 9c9b6c6 on master.

[andy-esch]

last_source needs to be returned here. The old code had:

temp.addTo(map, last_source)

There may be a better name than watername_ocean, or we should detect if it's a CARTO basemap -- and use watername_ocean if so, otherwise just plop it on the top.