Change `sphericalharmonics` to `spherical_harmonics` in data names

[vaitkus]

This is a low priority janitorial suggestion.

Is there a reason for naming all data items dealing with spherical harmonics as .*sphericalharmonics.* and not .*spherical_harmonics.* (e.g. _pd_pref_orient_sphericalharmonics.y_j instead of _pd_pref_orient_spherical_harmonics.y_j)? The second option seems a bit more readable and is more inline with the rest of the data names used in the dictionary.

Since these data items have only recently been introduced to the dictionary and are very unlikely to be used in data files, maybe the items could be renamed?

[jamesrhester]

Definitely could be renamed. I think the cutoff for data name changes should be when a dictionary is formally released. I've initiated a cif core release process just now, and once that is under way we should start a powder dictionary release that will include the QPA and preferred orientation work.

[rowlesmr]

It just happened to be the way I wrote it. No issue in changing it.