Graph/GfaIO.h: Permit an overlap of 0 bp

A CIGAR string of 0M indicates two immediately adjacent segments.
Graph/GfaIO.h: Separate name and sense with a tab
Graph/AsqgIO.h: Permit an overlap of 0 bp

Not strictly allowed by ASQG, but necessary for compatibility with GFA.
Graph/SAMIO.h: Permit an overlap of 0 bp

Not strictly allowed by SAM, but necessary for compatibility with GFA.