Validation plotting fixes and generalization
Validation plotting fixes and generalization

- Fix bug in indexing plots with only a single group
  (thanks to @ohofmann).
- Ensure non-labelled plots have same dimensions as those with labels.
- Generalize groups and categories depending on numbers of samples
  and callers to provide nice caller comparisons in addition to the
  current sample comparisons.

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