Connectivity Fill value Validation

[david-sh-csiro]

It would be useful to add warnings to emsarray when a given dataset has provided geometry and fill values that conflict with eachother. E.g. if you provide geometry with 2,000,000 nodes but your fill value is 999,999. This results in erroneous values when accessing the connectivity geometry.

This could either be done as a ConventionViolationWarning when loading the dataset or by returning false when using a has_valid_foo_connectivity such as has_valid_edge_face_connectivity.