WDL: add support for sub-workflows
WDL: add support for sub-workflows

Allow resolution of sub-workflows in WDL conversion and add additional
notes about mapping approaches.

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