Use local Refseq mirror for post-kraken2 accession querying

[Merritt-Brian]

Description of feature

Currently, you have to either pull (internet access required) taxa from NCBI post-kraken2, manually pull (also internet-requiring) AND/OR skip kraken2 and use a local reference FASTA file. However, some would want to just use a full Refseq set of FASTA files (many directories/subdirectories) and extract those for the realignment step.

Considerations:
Read the headers of all files is too unwieldy.
Consider supplying a mapping file of accession to file OR extract accession from the filename itself like GCF....fasta