Pisces: support for low frequency (<1.0%) variants
Pisces: support for low frequency (<1.0%) variants

Provides lower emit threshold and higher variant thresholds to trust high
depth bases for low frequency calling, based on Pisces tuning adjustment
suggestions:

https://github.com/Illumina/Pisces/issues/14#issuecomment-399756862

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