Limit RTG memory usage by limiting threads
Limit RTG memory usage by limiting threads

- Ensure standard memory usage behavior on larger machines. Thanks
  to @Lenbok for memory estimation.
- Do not yet change vcf-score-field for ROC creation but document
  discussion. @scatreux preferred a single metric for comparisons
  across multiple callers (QUAL) but @Lenbok suggested caller
  specific fields
  (https://github.com/chapmanb/bcbio-nextgen/commit/a538cecd86c0000d17d3f9d4f8ac9d2da04f9884#commitcomment-14601753)

Cyberbio-Lab/bcbio-nextgen,chapmanb/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,biocyberman/bcbio-nextgen,mjafin/bcbio-nextgen,lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen,chapmanb/bcbio-nextgen,biocyberman/bcbio-nextgen,brainstorm/bcbio-nextgen,a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,mjafin/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,brainstorm/bcbio-nextgen,lpantano/bcbio-nextgen,lpantano/bcbio-nextgen,brainstorm/bcbio-nextgen,gifford-lab/bcbio-nextgen,gifford-lab/bcbio-nextgen,a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,biocyberman/bcbio-nextgen,lpantano/bcbio-nextgen,mjafin/bcbio-nextgen,lbeltrame/bcbio-nextgen,gifford-lab/bcbio-nextgen