Minor improvements to DepthHistogramNode
Minor improvements to DepthHistogramNode

1. Check for failed commands when running, causing the node to abort
   if a command exists prematurely.
2. Improve filtering of reads that should not be included in depth
   calculation; alternative alignments, chimeric alignments, etc.
3. Minor optimizations.

MikkelSchubert/paleomix,MikkelSchubert/paleomix,MikkelSchubert/paleomix