Corvettes not selectable for auto-convoy

[DBS]

Relatively minor but... PG-class vessels are not eligible for inclusion into the auto-convoy pools.

I realise this may fall into the "works-as-designed" category, but if so it means that the Flower Class corvettes have been misgraded as PG. The corvette was designed for convoy escort duties, and in the Indian Ocean tended to conduct precisely the sort of convoy work covered by the auto-convoy system. Not sure either why the shorter-ranged Japanese PC are acceptable as escorts, but their larger PG vessels are not.