_diffrn_radiation_wavelength.type, xray_symbol

[James Hester]

Dear CoreDMG,

As per the issue raised at https://github.com/COMCIFS/cif_core/issues/168 ,
there are around 800 CIF files in the Crystallographic Open Database (COD)
that incorrectly loop _diffrn_radiation_type in the
_diffrn_radiation_wavelength loop, although the DDL1 (and DDLm)
dictionaries state that these data names belong to separate categories. The
reason for doing this is understandable - for example, if you want to
identify that there is contamination by Kbeta, then you might use two
distinct labels for _diffrn_radiation_type (K\alpha and K\beta) and
logically these would be attached to the particular wavelengths. I
interpret this as suggesting we need to improve this part of the dictionary.

Unfortunately we cannot just move _diffrn_radiation.type into the
_diffrn_radiation_wavelength category, where it probably should have been
from the start, because there are 230 COD CIF files where the authors have
done the right thing (insight from other databases welcome of course) and
making previously correct CIF files incorrect is Not What We Do.

Therefore I suggest we:
(1) add new datanames _diffrn_radiation_wavelength.type and
_diffrn_radiation_wavelength.xray_symbol to the diffrn_radiation_wavelength
category, with identical definitions to their current _diffrn_radiation
definitions
(2) deprecate _diffrn_radiation.xray_symbol and _diffrn_radiation.type

Thoughts?

James.
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