fixing parsing COMPND record, adding test cases and docu

modules/base/src/string_ref.hh

modules/io/src/mol/pdb_reader.cc

modules/io/src/mol/pdb_reader.hh

modules/io/tests/CMakeLists.txt

modules/io/tests/test_io_pdb.cc

modules/io/tests/test_io_pdb.py

modules/io/tests/testfiles/pdb/1AKE.pdb

modules/io/tests/testfiles/pdb/1AKE_noBchain.pdb

modules/io/tests/testfiles/pdb/3mk3.pdb

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

...ests/testfiles/pdb/compnd_missing_chains_chain_record.pdb

...tests/testfiles/pdb/compnd_missing_colon_chain_record.pdb

...ests/testfiles/pdb/compnd_missing_colon_mol_id_record.pdb

...ts/testfiles/pdb/compnd_missing_complete_chain_record.pdb

...s/testfiles/pdb/compnd_missing_complete_mol_id_record.pdb

...iles/pdb/compnd_missing_mol_id_complete_mol_id_record.pdb

...s/testfiles/pdb/compnd_missing_semicolon_chain_record.pdb

.../testfiles/pdb/compnd_missing_semicolon_mol_id_record.pdb