_struct_ref_seq category has .pdbx_strand_id but no .asym_id

[drlemmus]

The pdbx items of _struct_ref_seq use .pdbx_strand_id to reference specific macromolecular chains that are alignedto a reference sequence, but there is no .asym_id equivalent. This forces us to use 'PDB numbering' which seems to be a step back from the mmCIF idea.

1. Can the item _struct_ref_seq.asym_id be added?
2. Should the item _struct_ref_seq.pdbx_strand_id perhaps be deprecated?