Add explicit support to SeqDiff for insertions and deletions

[benmwebb]

SeqDif currently assumes that it will be used only to describe a mutation and so only references the entity seq_id. Add explicit support (perhaps with SeqDif subclasses) for deletions (which would need the reference database seq_id instead, and have no entity seq_id) and insertions (both generic insertions and expression tags) which have an entity seq_id but no reference database seq_id. Update our checks to take insertions/deletions into account too, and run the new code against all entries in current PDB-IHM. @brindakv is pretty sure that entries in regular PDB should all be correct, since these are generated by the sequence module, but we don't currently check PDB-IHM.

See also ihmwg/python-modelcif#54.