Prevent tRNAs from matching non-tRNA Rfam models

[AntonPetrov]

Currently URS000043457D matches sx4:

but in fact it is a really nice tRNA:

To avoid a match to sx4, one should have a tRNA model in the rfam.cm library, and then ignore anything that hits best to the tRNA model - since there is the tRNAscan-SE step later.