Use more precise SO terms for mt rRNAs

[blakesweeney]

We are looking at the metadata file for CRW now (https://github.com/RNAcentral/R2DT/blob/master/data/crw-metadata.tsv) and it would be helpful if the models had more precise terms. Specifically there are several models labeled mt_rRNA. It would be better to use the LSU vs SSU term whenever possible. Judging by the naming I think all *.16.* models should be mt_SSU_rRNA, while the result should be mt_LSU_rRNA.