map MGI organism IDs to NCBI taxon ids

the taxons listed in the MGI genotype table are actually MGI's internal organism identifiers and labels.

these need to be mapped to proper ncbi taxon ids (which is the standard we are mapping everything to in other tables.) we could keep the mgi's internal organism identifiers as a separate col, as might be useful as an additional description of the organism.

i believe there is no internal id mapping in any of the mgi base tables to NCBI taxon identifiers, so these will have to be handled by hand with a case statement.