CWL: Improvements for joint calling runs
CWL: Improvements for joint calling runs

- Include enough disk space allocation for `to_rec` functions based
  on larger sample size testing.
- Ensure allocation of cores to `prep_samples` to take advantage of
  multicore BAM or fastq conversion.
- Avoid running joint calling finalization without batches, just
  produce gVCF.

a113n/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen