Custom colour maps

[burggraaff]

I've added the ability to use different colour maps. While the default is great most of the time, this change should allow for more customisation without adding too much complexity.

The colour map can be selected in the terminal using the -c or --color-map argument with the name of the colour map. By default, the map will be imported from branca (which folium requires, so no new dependencies). Additional options are matplotlib (mpl.<name>) and cmcrameri (cmc.<name>) – these are only imported when used, so it only adds optional dependencies. The selected colour map is converted into a gradient of hex strings, because this is what the folium HeatMap plugin requires; support for directly passing a colour map object should be added to folium at some point.

Some examples:

1. Default (no change)
   

2. Paired_12
   

3. Set1_09 with blur -b 2
   

4. mpl.cividis
   

5. cmc.lajolla with blur -b 2