cif_img.dic: standardise the "photons per second" unit name and describe the unit in the ITEM_UNITS_LIST loop

[vaitkus]

Issue was noticed in the cif_img.dic dictionary, version 1.8.5.

The photons per second unit appear a single time in the dictionary in the definition of the _diffrn_radiation.beam_flux data item. The usage of this item has two minor issues:

• Unit name photons per second uses spaces to separate words while all other units from the dictionary use underscores (e.g. degrees_squared, counts_per_photon). It would be more consistent to rename it to photons_per_second.
• All units used in the dictionary are described in the ITEM_UNITS_LISTloop (_item_units_list.code, _item_units_list.detail) except for the photons per second. The missing description should be added.

I can create a relevant PR if the proposed changes are acceptable.

[vaitkus]

The CBF/imgCIF dictionary seems to now have been split into a separate repository [1].

This issue was resolved in the draft dictionary version 1.8.8 located in the aforementioned repository.

[1] https://github.com/yayahjb/cbf_imgcif_dictionary/tree/main