parsing COMPND records, supported are MOL_ID and CHAIN records only

modules/io/src/mol/pdb_reader.cc

modules/io/src/mol/pdb_reader.hh

modules/io/tests/test_io_pdb.cc

modules/io/tests/testfiles/pdb/compnd.pdb

...tests/testfiles/pdb/compnd_missing_comma_chain_record.pdb