Nautical miles scale

[benoit-jpi]

a nautical mile scale should replace the statute mile scale on the map.

[claustres]

We now use the standard Leaflet scale control that does not provide it out-of-the-box. Maybe a good idea to check if https://github.com/jtreml/leaflet.customscale could be used instead, or be directly implemented in KDK as the code is pretty old.

[claustres]

Should now be more easily possible to integrate https://github.com/kalisio/leaflet-graphicscale thanks to kalisio/kdk#818.