Embed all required map data in application

[kellpossible]

#65 has prompted me to re-visit the topic of removing the external service dependency for map rendering.

We could perhaps use the following tools to do this:

• https://github.com/protomaps
• https://github.com/maplibre/maplibre-gl-leaflet

The only problem is how to render contours, potentially we can render them ourselves using the height map that is required for #39

[kellpossible]

Alternatively I could provide a way to serve tiles downloaded from MapTiler https://documentation.maptiler.com/hc/en-us/articles/4404773141649-How-to-download-data-from-MapTiler-website

[kellpossible]

#2 is another alternative for OpenTopoMaps at least. Probably the simplest option.