perceptually uniform colormap for Heatmap

[FelipeLema]

I call for an upgrade in colormap/gradient used in Heatmap. Several plotting libraries/packages have followed this trend of having viridis as default. (incl. Matplotlib, MATLAB and plotly)

More details of the reasons why here.

[martinRenou]

I agree. I think at some point ipyleaflet will use bqplot's scales for the Heatmap and the Choropleth layers, which comes with a viridis colormap. And we could use the viridis one by default.