Add API support for SA tags

[jamesemery]

The SA tag field for reads is becoming more relevant with the introduction of alternate contigs into references which make use of chimeric alignments to denote reads aligned to an alternate contig. The SA tag, which is necessary to distinguish secondary alignments, is defined in the SAMSpec as follows:

Other canonical alignments in a chimeric alignment, formatted as a semicolon-delimited list: (rname,pos,strand,CIGAR,mapQ,NM ;)+. Each element in the list represents a part of the chimeric alignment. Conventionally, at a supplementary line, the first element points to the primary line.

These are complicated enough to get right that there needs to be code that deals with them as more than just strings. Some of the desired utility is added by htsjdk but this only allows for reading the SA tag, not altering it.