Support shortnull

Implements shortnulls behavior with new config variable set to True by default.

This will address things like DRB4*01:03N and DRB5*01:08N both in terms of accepting them as valid and also by expanding them as appropriate to the list of longer alleles that have the same expression character.

DRB5*01:08N is acceptable by WHO rules since all suballeles have N.
DRB4*01:03N is acceptable by WMDA (but not WHO) rules but this needs to be handled for things like HF analysis by expansion to something valid (vs rejection) to avoid bias