Layers as xarray.DataArray in split_map (ipyleaflet Map)

[lopezvoliver]

Following #680, here's a PR that implements the ability to pass left_layer and right_layer as xarray.DataArray in split_map, here only for the ipyleaflet.Map version for now.

[giswqs]

Can you also overrite the 49_split_control.ipynb under docs/notebooks, which is used for building the docs website? For now, leafmap has duplicate copies of notebooks under examples and docs. I should probably get rid of the examples folder entirely in the future.

[giswqs]

The docs-build failed because the notebooks are executed with the folium backend. Would you be able to make similar changes to the split map function in foliumap.py?

[lopezvoliver]

Not sure why the docs-build test failed on notebook 83_vector_viz.ipynb

[giswqs]

The notebook issue seems caused by the recent updates of the lonboard package. I have reported it at developmentseed/lonboard#365. Ignoring this notebook for now.