hmm_io.read fails if COMPO line is missing

[sbliven]

TRAL fails to read HMMER3 files that omit the optional COMPO line at the beginning of the model section.

According to the spec, the model section starts with an optional composition line, then the I_0 emission probabilities and the BEGIN transistion probabilities. Instead, TRAL treats these as a normal (match/insertion/transition) state and the state machine requires all three to progress.