Add an example demonstrating multiple CRS support

[jailln]

Add an example demonstrating that itowns supports multiple CRS by e.g. allowing to switch CRS with a checkbox, something like the recent new demo of mapbox: https://docs.mapbox.com/mapbox-gl-js/example/projections/

By the way, an interesting blog post on non mercator web projections in interactive web maps has been published by mapbox: https://www.mapbox.com/blog/adaptive-projections where some interesting adjustments are described. I think they should be discussed / considered for itowns.

My opinion on these:

• automatic rotation for north up aligment: may be proposed as an optional feature
• distorsion correction when zooming in: this takes full potential of interactive maps but does not preserve the characteristics of the chosen projection, so I think that depending on what you are doing it is not always what you want to have
• resampling vector geometry : useful for when mvt will be rendered as vectors in itowns
• their way to reproject raster tile: how could MARTINI benefit to itowns ?

The PR in mapbox: https://github.com/mapbox/mapbox-gl-js/pull/11124/files

[gchoqueux]

I open a conversation #1777 on this next feature.

[jailln]

Until we create an example demonstrating this, there is a tutorial explaining how to do it