clinseq-converge-stats update

Add a new file to the converge stats command, this is the Stats.tsv from summarizing SNVIndelReports. To help accomplish this make base quality and mapping quality a parameter of Base.pm under Converge.
`File::Spec->join` is platform independent (not that its particularly likely that we'll be porting this to Windows any time soon) and it tries to be smart about trailing and leading `/` when concatenating parts of the path.