Allow Case=Voc with adjectives

[kscanne]

It's a small thing, but there are six adjectives in the vocative case in the treebank. Presently, these don't have a Case feature since this isn't currently a permitted value according to the validator. For consistency, it would be nice to add these Case=Voc features.

First example is in sent_id=508 in the dev file:

1   A   a   PART    Voc PartType=Voc    2   case:voc    _   _
2   mhná    bean    NOUN    Noun    Case=Voc|Definite=Def|Form=Len|Gender=Fem|Number=Plur   10  vocative    _   _
3   uaisle  uasal   ADJ Adj Gender=Fem|Number=Plur  2   amod    _   _
4   agus    agus    CCONJ   Coord   _   6   cc  _   _

...

[kscanne]

Actually, it looks like this is a permitted value, sorry about that. So if @tlynn747 is onboard with this change, I can just make it directly to the treebank with no additional changes required.

[tlynn747]

I'm onboard