Validation findings should have problem codes along with problem descriptions

[frizbog]

The problems returned from GedcomValidationFinding includes textual descriptions of the problems. This is not really a problem on its own, but there should also be codes that identify the problem being described, so users can supply alternate descriptions if they want, and changes to the descriptions would not break code looking for particular kinds of problems.

The codes must not be an enum value; this would prevent users of gedcom4j from adding their own validations and codes. The code could be a copy of the orginal value or a textual code in an enum of built-in codes, but values other than those in the enum must be allowed.

[frizbog]

It will be important not to change the order of the enum constants once they start getting used.