Explicitly specify version numbers of imported dictionaries

[vaitkus]

The powCIF dictionary currently imports the imgCIF, multiCIF and coreCIF (indirectly) dictionaries. It would be nice to ensure that these dictionaries also have stable releases which we can explicitly mention in the import statements. Of course, software if free to ignore these version numbers (mine currently does), but at least this would serve as an indicator for the users that "the dictionary was developed and checked with these specific dependencies in mind".

I understand that this might be too much for this initial release, but I decided to at least raise this idea for further discussion.

[vaitkus]

There are two related issues/PR regarding the naming of the multiCIF dictionary:

• Renamed dictionary to CIF_MULTI MultiBlock_Dictionary#35
• Rename the dictionary file from multi_block_core.dic to cif_multi.dic MultiBlock_Dictionary#36

It would be nice to resolve them before releasing the pdCIF dictionary since the dictionary file and block names are explicitly mentioned in its import statements. It would be preferable to not change these name too much in future releases.