Import: categorical fields - optional behaviour to flag casting fails rather than propagate exception

[atbenmurray]

For categoricial fields, there should be an option to flag unrecognised values rather than raising an exception and stopping import. It would nice to report that unrecognised values were found.

Alternatively, we can have the option to add them to the key for that categorical value but that comes with issues such as what happens if you attempt to reconcile two datasets where the same values are seen in different orders.

This is an issue for scripts that assume they understand categorical values directly rather than always looking up the keys.