Gracefully accept either NetCDF longitude std name.

See #669 and #703 for further details and discussion.

Relaxing the standard name condition given the units of ```degrees``` for a longitude coordinate is exercised by multiple existing unit tests, so I don't see the benefit of including an explicit test for this PR.

This PR relaxes the standard name criteria for identifying a longitude coordinate with units of ```degrees```.

The CF specification implies that **only** a standard name of ```grid_longitude``` should be associated with a rotated pole units of ```degrees```.