Make more use of xarray's decode_coords='all'

[dopplershift]

In pydata/xarray#2844, the option decode_coords='all' was added, which handles turning the appropriate grid_mapping variables into coordinates at dataset load time. This came along after we built up a bunch of our xarray integration, so there are some things I wonder now:

• Could we deprecate our metpy_crs coordinate in favor of relying on decode_coords='all' and doing everything else we need with projections on-demand? Might be some issues with our inference of lat/lon projections.
• The need to work across multiple, non-coordinate variables was the main motivation for the parse_cf() approach in the beginning. Could we deprecate parse_cf() entirely and do everything else on demand? The "special" handling for GOES coordinates is still something that might be tricky.

#3664 is handling making sure we work with this option at all, since it breaks our own finding of grid_mapping in parse_cf().

cc @jthielen