FastaSeqFetcher improvements

[davmlaw]

cdot provides a lot of transcripts that aren't in seqrepo

So, you basically always want to use FastaSeqFetcher

We should add this to the docs

We should also detect when it's a good idea or not - ie Ensembl is fine, but with RefSeq, we could store the mismatches or the notes in the GFF about it having mismatches, and then either warn or refuse to map (maybe via setting?)

Storing mismatches might require a version bump to change formats, or we could store as separate field than the cigar