The commercial simulator does not support parenting a well directly
to the FIELD group and Flow in multiple locations implicitly assumes
that no well is parented directly to FIELD either.
With this commit we detect this situation at the parser stage and
stop reading the input file. In this case, we issue a diagnostic
message of the form
Error: Problem with keyword WELSPECS
In SPE1CASE1.DATA line 384
Wells cannot be parented directly to 'FIELD'.
Wells entered with disallowed 'FIELD' parent group:
* PROD
* INJ
Error: Unrecoverable errors were encountered while loading input