Updated version of DDLm core cif after merging symmetry dictionary

[James Hester]

Dear Core CIF DMG members,

I have completed merging the symmetry dictionary into the draft DDLm core
CIF dictionary with advice from David Brown. To avoid cluttering your
mailboxes with 1M files, the link to the latest version is:

https://github.com/COMCIFS/cif_core/blob/cif2-conversion/cif_core.dic

Unfortunately there are too many changes for the Github automated 'diff'
listing of the changes to be provided. The important changes can all be
found following the 'SPACE GROUP' category, and see my notes below.

A new 6-week review period starts today.  As before, please check those
areas of the dictionary that time and your expertise permit, and start a
new mail thread for each distinct topic to help manage the conversation.
Even if you find no issues, a public message that you have conducted a
review and are satisfied is better than silence.

James Hester (acting Core DMG chair).


Notes on the symmetry dictionary merging:
==================================

(1) Categories 'symmetry' and 'symmetry_equiv' have been changed to
'space_group' and 'space_group_symop' respectively, in line with the names
used in the symmetry, modulated structures, and forthcoming magnetic
dictionaries.
(1.1) Where the part of the name following the category name differs, the
symmetry dictionary version was chosen.
(2) Category 'space_group_Wyckoff' has been transferred across from the
symmetry dictionary
(3) New category 'space_group_generator' has been created as the space
group generators should not be looped together with the symmetry operators.
(4) Category 'space_group' is now not loopable, that is, only a single
space group can be provided in any data block (as was the case for the
'symmetry' category). If this wrecks your use case, please start a
conversation.
(4.1) Any datanames used to identify a particular space group from the loop
of space groups have therefore been dropped as they are now unnecessary
(5) Datanames from the symmetry dictionary linked to the reference setting
have not been merged as, to be useful, they would require new datanames for
every affected concept e.g. atom_site.pos_x_reference, refln.hkl_reference.
These would only clutter up the core dictionary but might be usefully
defined in a reborn symmetry dictionary if anyone wants to volunteer.

-- 
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