[ENH] density methods

[ljwolf]

This would be a solid addition to plotting in Python and I think we can get a kde map by cleaning up a Geoandas dataframe and then using the existing two dimensional kde from seaborn on centroids.

I'd also like to get legendgram in.

Also hexagonal histograms would be easy to integrate.

[ljwolf]

If there is a "roadmap" tag, this could be tagged for that?

[ljwolf]

https://kdepy.readthedocs.io/en/latest/ looks much faster than existing seaborn methods for big-N heatmapping.

[darribas]

Nice! we could consider making kdepy a requirement for splot but fall back to scipy/seaborn if not present.

[knaaptime]

superseded by pysal/pointpats#118