Editor: GMST verifier
Verify the correctness of GMSTs. May require new stage subclasses.
Empty strings should be seen as an error.
The range of valid values for numeric GMSTs needs to be determined by examination of MW and OpenMW.
(RM-652 from redmine: created on 2013-03-21 by Marc Zinnschlag, , closed on 2016-01-19 by Kyle Cooley)