fix backbone trace creation
fix backbone trace creation

The backbone trace creation was failing when a single amino
acid was followed by a non-aminoacid residue, which was
then again followed by an amino acid.

Traksewt/pv,biasmv/pv,biasmv/pv,CyrusBiotechnology/pv,biasmv/pv,CyrusBiotechnology/pv,Traksewt/pv,biasmv/pv,yuhangwang/pv,yuhangwang/pv,CyrusBiotechnology/pv,yuhangwang/pv,yuhangwang/pv,CyrusBiotechnology/pv,Traksewt/pv,Traksewt/pv