Relax NetCDF longitude circularity criteria.

The current criteria for detecting NetCDF longitude circularity is unnecessarily restrictive.

This PR relaxes the current requirement on the candidate coordinate name. This change allows a longitude coordinate that has been identified via the miscellaneous ```fc_default_coordinate``` PyKE rule (i.e. no ```coord_name``` is passed to the Python ```build_dimension_coordinate``` convenience function) to be correctly identified as circular in nature.

Interestingly enough, I did expect unittest failures due to this PR change ... but the CML does not capture the state of the ```circular``` flag for a ```DimCoord```.

Whether we want to do this is another matter, and if we do, I suggest this is the action of a separate PR.

Requires to be rebased once PR https://github.com/SciTools/iris/pull/693 is accepted and merged to upstream/master.