parse posterior probabilities in stockholm files

Stockholm files support 'PP' or 'posterior probability' in GR annotation lines to encode local model fitness posterior probabilities:
From http://sonnhammer.sbc.su.se/Stockholm.html

PP Posterior Probability [0-9*]
              (0=0.00-0.05; 1=0.05-0.15; *=0.95-1.00)

(GC lines allow PP_cons).

These are output by other tools, including RNAlien (see e.g. http://rna.tbi.univie.ac.at/rnalien_static/cm26140/result.stockholm ).