Automatically define {AREA_SIZE}'s value etopo|srtm250|srtm90

[hugolpz]

• rename into something such as variable gisdata_(quality)_required
• create a calculative function and conditional assignation

See files :

./master.makefile                         # set by default with "export AREA_SIZE=etopo"
./script/get_raster_source.bash   # nothing calculated here !