Make WbPlanView better detect if a field is required or not

[maxpatiiuk]

Some fields are marked as required, yet do not actually prevent an upload (Locality > Locality Name)
Some fields are marked as not required but would prevent upload if not mapped (Collection Object > Catalog Number)

Knowing whether a column is required is more complicated than it seems and the field being marked as required or not required may not always align with the real expectation of the uploader (due to business rules).

For example, Latitude/Longitude Type is not required, yet must be provided if Latitude2/Longitude2 is provided.
Similarly, a Catalog Number is not required, but it must be unique, which is only possible when the value is provided.

Also, required often simply means that a field must have a value before it is stored, but it doesn't tell whether the value has to be supplied by the user or by the uploader.

For now, the best way to find out if a field is required is to run validation.

[maxpatiiuk]

Another case:
If you are matching against an existing record in a table, rather than creating a new one, then required fields don't have to be present, as long as the present fields can uniquely identify the record you are trying to match.

This is why if you set a table as "must match", all of its required fields start to show up as optional