Adds basic multiprocessing support.
Fixes: Breaks when excluding taxids and there are already flat records with that species.
Marks extracted records to NOT re-extract.
Improves identity calculation in the 'flatten' step.
Distributes workload between CPUs.
Removes user choice when there is more than one feature within a record matching locus specifications. This will probably cause issues when there are genuine multiple loci within a record.