Add the `_atom_site_fract.symmform` and `_atom_site_aniso.symmform` data items

[vaitkus]

This PR adds the _atom_site_fract.symmform and _atom_site_aniso.symmform data items (see discussion in [1]).

The definitions of these items were almost verbatim copied from the document attached in [1] with the following changes:

• Data item _atom_site_fract.symmform was renamed to _atom_site.fract_symmform (changed the placement of the dot). The category of this item was also changed from ATOM_SITE_FRACT to ATOM_SITE.
• The human-readable definition of _atom_site_aniso.symmform was changed to use the term "anisotropic displacement parameters" instead of "anisotropic thermal ellipsoid".
• Both items were assigned the "Assigned" _type.source.
• Both items were assigned the "Encode" _type.purpose. (I assume here that they are intended to be machine-parsable).

Before merging this, please also note that @jamesrhester raised some reasonable concerns in [1] (e.g., the same information might be better conveyed using a CIF2/DDLm list). However, since the _atom_site_fract.symmform is already used by some pieces of software and already appears in the wild, I suggest that we retain the semantics of these two data items and introduce additional list items (now or later) if so desired. How does that sound?

[1] https://github.com/COMCIFS/magnetic_dic/issues/59

[vaitkus]

Thank you for the review.

The issue that deals with the list form of these items is #484, the same will probably have to be done in the magnetic CIF dictionary as well.