Rename Head category to 'CIF_CORE_HEAD'

[vaitkus]

This PR renames the Head category as discussed in issue #488.

Note that since the Head category name is used in the import statements of multiple dictionaries, these should be updated as soon as possible after merging the PR. I will provide a list of PRs for the dictionaries in question in a separate comment.

The ATOM_SCAT_VERSUS_STOL data item update dates were changed since I somehow managed to set them a month in the future in my previous PR.

[vaitkus]

PRs in other repositories that should be merged as soon as possible after merging this PR:

• Update CIF_CORE import statement Powder_Dictionary#170
• Update CIF_CORE import statement Electron_Density_Dictionary#13
• Update CIF_CORE import statement Modulated_Structures#24
• Update CIF_CORE import statement Restraints_Dictionary#4
• Update CIF_CORE import statement Twinning_Dictionary#2
• Update CIF_CORE import statement TopoCif#81
• Update CIF_CORE import statement MultiBlock_Dictionary#14
• Update CIF_CORE import statement cif_ed#8

There is also the automatically generated imgCIF dictionary (https://github.com/COMCIFS/imgCIF), but it seems to refer to a fixed CIF_CORE version.