Add cifs for every commonly used NIST SRM

There should be cifs added that are compliant with the NIST SRM's, such that they can be used for refinement. The lattice parameters and atomic positions must match those defined in the NIST Standard. Examples include:

Si, LaB6, CeO2 and any others used regularly by the crystalllography beamlines 660a, 660c etc

Start by locating how existing CIF files are stored and loaded in xrpd-toolbox, then verify the NIST SRM lattice parameters and atomic positions for Si, LaB6, and CeO2. Done means the requested commonly used standards are available as refinement-ready CIFs and their values match the NIST standards.

Written by the indexing model from the issue text.