Bug Report for error when a PDB is missing a chain ID


**description:** missing a chain leads to error 

**Error:** with ref files containing 5 chains, but a PDB containing only 4 chains and missing chain D stops the program to run
while ***running pdb_match_chn_batch.py*** line 62
line 62: ***PDB_chain -D ..***
is stopped

**Expected:** Missing a chain is no problem and would still include that PDB to be matched without error.