alter key pd_qpa_external_std

[rowlesmr]

pd_qpa_external_std - originally keyed on .diffractogram_id. This was only used to get to the associated .instr_id value, so I decided to cut out the middleman and key it directly on .instr_id. This has the side-effect of requiring a new _pd_instr.id if a recalibration is required, but this was always implicitly there. .diffractogram_id demoted to linked, non-key data name. .instr_id added as category key. Updated the example to link the mass absorption coefficient of the specimen to the diffractogram though a pd_char, pd_prep, pd_spec chain.