ud2gf: Fallback to morphoanalyze if parse fails + use Str functions for lexical backups

[anka-213]

• If a lemma fails to parse with gf in the expected category, check if morphoanalyze gives a result.
• If neither succeeds, instead of inventing new imaginary lexical functions, look for functions of the shape

Str[ExpectedCat] : Str -> ExpectedCat

This is a somewhat breaking change if someone relied of the old way of inventing functions, but on the other hand, it didn't produce valid GF expressions in those cases, so it's unlikely that anything would break in practice.

One option is to put this behaviour behind a flag, so the new behaviour is opt-in, as is suggested in #24?