Harmonise dictionary naming conventions

[vaitkus]

See issue COMCIFS/cif_core#488 for the full discussion.

To keep in line with the historic dictionary naming conventions the following changes should be made:

• Rename file cif_pow.dic to cif_pd.dic (this is the name under which it appears on the IUCr website).
• Change dictionary title and all related block names, etc. changed from CIF_POW to CIF_PD.
• Rename PD_GROUP block to CIF_PD_HEAD.

[jamesrhester]

Agree.