Reduce number of dependencies

[gab23r]

I am surpised to see the number of "heavy" dependencies of ipyleaflet.
I think that most of them are not required ( scipy, bqplot, geopandas, xarray )

I think it would be an improvement to remove those from the environment.yml, so that user can make a conscious choice to add them or not in their environnent.

[martinRenou]

ipyleaflet only requires ipywidgets, branca, traittypes and xyzservices which are relatively small dependencies and do not pull the packages you mention (see https://github.com/jupyter-widgets/ipyleaflet/blob/master/setup.py#L58).

You are looking at the environment file needed for running the examples on the repository, not the dependencies of the package itself.