Structured CIGAR; added barebone GADataSet

It is now possible to describe the entire chimeric alignment in one GARead record. In case of a graph reference, each linear alignment could be an alignment to a segment in the graph. Note that in GACigarOperation, I am using single-character values. I believe this is easier for people who routinely work with the SAM format.

This pull request addresses #8, #9, #22 and #30.