Max number of individual readgroups for samtools stats

[sorelfitzgibbon]

          Some of the GIAB samples for example have > 1000 readgroups.  I think I should impose a maximum number of readgroups, and I suppose libraries, that will be processed separately by `SAMtools stats`.   But for `FastQC` they will all be processed separately as this is equivalent to running `FastQC` on the FASTQ files.

Originally posted by @sorelfitzgibbon in #62 (comment)