Added group identifier type validation
Added group identifier type validation

Also fixes for
https://github.com/ebu/ebu-tt-live-toolkit/pull/395#pullrequestreview-30
267500.

Fixes #401

bbc/ebu-tt-live-toolkit,ebu/ebu-tt-live-toolkit,ebu/ebu-tt-live-toolkit,ebu/ebu-tt-live-toolkit,bbc/ebu-tt-live-toolkit,bbc/ebu-tt-live-toolkit