add _diffrn_refln.diffrn_id

[rowlesmr]

will close #51

Added _diffrn_refln.diffrn_id as linked key, forming composite key.

Required to enable the following missing link:

  !  FK: '_diffrn_refln.wavelength_id' -> '_diffrn_radiation_wavelength.id': partial FK to 'diffrn_radiation_wavelength' -- covers ['id'] of PKs=['id', 'radiation_id'], no transitive bridge found -- skipping FK constraint

without any jiggerypokery,