Finished handling of SEQRES by the MMCifParser, with testing and documentation.

modules/io/pymod/__init__.py

modules/io/pymod/export_mmcif_io.cc

modules/io/src/mol/mmcif_reader.cc

modules/io/src/mol/mmcif_reader.hh

modules/io/tests/test_mmcif_reader.cc

...s/io/tests/testfiles/mmcif/changing_label_entity_id.mmcif

modules/seq/alg/doc/seqalg.rst

modules/seq/alg/pymod/__init__.py

modules/seq/alg/tests/test_aligntoseqres.py

...eq/alg/tests/testfiles/validate_segres_aln_breakage.mmcif

...q/alg/tests/testfiles/validate_seqres_aln_connected.mmcif