#995 Add '@@partial' annotation to CIF

• Best to review per commit.
• End-user visible changes:
  • CIF now features the @@partial built-in annotation. It can be added to partial specifications to suppress certain warnings that are often false positives for partial specifications. See the language tutorial and language reference manual for further information.
  • In the documentation about built-in annotations, in the CIF tutorial and CIF language reference manual, there are now no more annotation categories, just single annotations. We don't have any categories with multiple annotations, and this is just simpler and more direct.
  • In the documentation about built-in annotations, in the CIF language reference manual, pages now have additional sections.
• Note that tools other than the CIF type checker have not been updated for this new annotation. That will be done in a follow-up merge request.

Addresses #995 (closed)