Merge branch 'mmcif_writing' into develop

modules/io/doc/io.rst

modules/io/doc/mmcif.rst

modules/io/pymod/__init__.py

modules/io/pymod/export_mmcif_io.cc

modules/io/src/mol/CMakeLists.txt

modules/io/src/mol/mmcif_str.cc

modules/io/src/mol/mmcif_str.hh

modules/io/src/mol/mmcif_writer.cc

modules/io/src/mol/mmcif_writer.hh

modules/io/src/mol/star_writer.cc

modules/io/src/mol/star_writer.hh

modules/mol/alg/doc/molalg.rst

modules/mol/alg/src/biounit.cc

modules/mol/base/src/chain_type.cc

modules/mol/base/src/chain_type.hh

modules/mol/base/src/chem_class.hh

modules/mol/base/tests/test_chain.cc