Added support for _space_group.name_H-M_ref keyword in CIF files

[rozyczko]

This keyword is a tightly defined version of the keyword which can be used to generate the symmetry operations of the reference setting, but only one form of this symbol (that given in the list of values specified by the dictionary) can be used for a given space group.

International Tables for Crystallography Volume G: Definition and exchange of crystallographic data
Series: International Tables for Crystallography G
Author(s): S. R. Hall, B. McMahon (auth.), S. R. Hall, B. McMahon (eds.)
Publisher: Springer Netherlands
Year: 2005
ISBN: 1402042906; 9781402054112; 1402054114;

p. 461

[sbillinge]

thanks @rozyczko ! We don't have CI running atm. Please can you confirm that all tests are passing and I will merge.

[rozyczko]

Running them locally, windows 10/x64

(p3.8) C:\Users\piotr\projects\temp\diffpy.structure>python -m diffpy.structure.tests.run
..........................................................................................................................................
----------------------------------------------------------------------
Ran 138 tests in 1.124s

OK

(p3.8) C:\Users\piotr\projects\temp\diffpy.structure>git branch
* HM_ref_support
  master

(p3.8) C:\Users\piotr\projects\temp\diffpy.structure>git remote -v
origin  https://github.com/easyScience/diffpy.structure.git (fetch)
origin  https://github.com/easyScience/diffpy.structure.git (push)

I didn't put structure under CI, counting on pypi distribution but we have full GH CI running for pdffit2.
https://github.com/easyScience/pdffit2

If you'd like, I can do a PR for the workflow files (and a fix or two)

Thanks!

[sbillinge]

Thanks @rozyczko !

A PR with fixes would be great. We are in the process of moving all our CI to GH Actions atm so it may not be the best time to add travis workflows. This decision was made to reduce maintenance overhead keeping ci going on all the different projects in the group, which before were using travis and circle and different platforms, and wanting it to be all the same syntax etc., not that we don't love Travis. Pavol didn't have ci running when he was maintaining the diffpy code because he was very careful himself with running tests before merging but with us maintaining and more different groups contributing (we hope!) it is time to sort this out.....

[sbillinge]

Thanks so much for your contributions, it really is appreciated, please keep them coming!