Fail-safe mechanism for SIFTS mapping using a configurable PID threshold

SIFTS mapping are sometimes wrongly generated for a number of reasons. For instance:
   - PDB entries with chain(s) containing single residue
   - PDB entries with chain(s) containing negative resNums
   - PDB entries with chain(s) containing residues with insertion codes
   - PDB entries with chain(s) containing residues with multiple variants for the same position
In most of the aforementioned cases, the PID of the generated SIFTS mapping is very low (usually under 20%). Hence, there is need to automatically detect such cases and fall back to NW alignment based mapping.