Samtools Idxstats version check

Only evaluate r### format samtools version numbers.  Newer versions contain Idxstats sub-command.

There might be better ways, but hoping this is a quick fix for this specific problem.
Should we just crash instead of using a newer version? It seems dangerous to print a warning and then just swap the version numbers. For instance, if this was called from inside a build, the version of samtools on the processing profile would now be a lie.