Add LISA colormap as an accessible method

[darribas]

It'd be great to add a matplotlib colormap object that takes the values from Moran_Local and returns the colors so the color coding can be passed when plotting LISA results in other contexts (eg. data in xarray structures). This would ensure that values of 0, 1, 2, 3, 4 are the same colors as in the lisa_clustermap plot.