variant calling: handle single base pair regions
variant calling: handle single base pair regions

Avoids removing single base pair target regions during sanity checking and
cleanup. This allows use for fingerprinting applications where we call
at a single position. Thanks to Mike Cariaso

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