Add support for alignments without a score #49
Description
Example file: SODC_model_05.cif.gz
In ma_alignment_details, the scores (and seq. identity) are optional. Currently, python-modelcif does not allow to read/write (or process with make_mmcif) a file which lacks _ma_alignment_details.score_....
This is probably related to modelcif.alignment.Pair not supporting score=None (which @bienchen had observed when needing to write files like the one linked above). If that is the case, it's probably best to also add support for identity=None since seq. identity info is also optional in ma_alignment_details.