support for HET records in PDB files

modules/io/pymod/__init__.py

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/het.pdb

modules/mol/base/pymod/export_residue.cc

modules/mol/base/src/impl/chain_impl.cc

modules/mol/base/src/impl/residue_impl.cc

modules/mol/base/src/impl/residue_impl.hh

modules/mol/base/src/property_id.cc

modules/mol/base/src/property_id.hh

modules/mol/base/src/query_state.cc

modules/mol/base/src/residue_base.cc

modules/mol/base/src/residue_base.hh

modules/mol/base/tests/test_entity.cc