SCHWED-4696: Read pdbx_entity_branch for a better chain description

modules/io/src/mol/mmcif_reader.cc

modules/io/src/mol/mmcif_reader.hh

modules/mol/base/doc/entity.rst

modules/mol/base/pymod/export_chain.cc

modules/mol/base/src/chain_type.cc

modules/mol/base/src/chain_type.hh

modules/mol/base/tests/test_chain.cc