SCHWED-4848: Function to get chain names of branched entitis

modules/io/doc/mmcif.rst

modules/io/pymod/export_mmcif_io.cc

modules/io/src/mol/mmcif_info.cc

modules/io/src/mol/mmcif_info.hh

modules/io/tests/test_io_mmcif.py

modules/io/tests/test_mmcif_info.cc