[python] switch between all region bands

skip the bands without data (or monochrome)
[doc] add note about contrib in readme
[doc] Markdown-ify the readme and add license
[weight] remove last get_*_pt()
[gdal] add handy method to display a raster
[nav_graph] get_vertex -> get_vertex_or_create to avoid confusion
[nav_graph] load if unknown, then weight is max

in order to allow exploration plan in unknown areas
[visibility] add visibility_map (wip)
[cmake] fix gladys.pc shall NOT require GDAL

Since GDAL.pc does not exists...
NOTE: ask GDAL to use pkg-config !!! :-)