add map_proj = 6 (Lat-lon)

[matthiasdemuzere]

• Addresses #199
• Allows to map WRF information that have MAP_PROJ = 6 (Lat-Lon)
• No test has been added.
• I also added this same reprojection information in wrftools.geogrid_simulator(). Also this is not tested?

A small test file is available here: geo_em.d04_map_proj6.zip

import salem
ifile = "geo_em.d04_map_proj6.nc"
ds = salem.open_xr_dataset(ifile)
ds.salem.quick_map('LU_INDEX')