lat/lon grids
The Grid2DObjects should be extended to be either Cartesian or lat/lon. Then grids that are read in lat/lon, for example from the NetCDF plugin, would be kept that way, until either points are extracted (then at the right place, even for polar grids) or until the grids would be properly reprojected. When writing a grid out, we could keep trace of its potential lat/lon origin in order to reproject it exactly like it was in lat/lon (for example, a TA grid that is based on a lat/lon DEM).
The grid projection could be either based on PROJ4 or an internal implementation.