R-scape improvements

[AntonPetrov]

• [x] indicate how many Rscape-significant base pairs are not shown in the displayed structure. This would be both a quality control hint (if an Rfam curated structure was messed up and missing a bunch of base pairs), plus an indication of whether a pseudoknot might be present.

• [x] include a note of how many Rfam-annotated base pairs are not shown in the Rfam structure because they're an annotated pseudoknot.

• [x] display pseudoknots. Perhaps draw a bracket over a contiguous string of nucleotides that are one side of the PK stem and lettering the two halves A and a, B and b.

[AntonPetrov]

Fixed in the latest version of R-scape that supports pseudoknots.