Update use of diffpy.structure to silence warnings from 3.4.0

[hakonanes]

The diffpy.structure maintainers renamed a lot of methods in yesterday's 3.4.0: https://github.com/diffpy/diffpy.structure/releases/tag/3.4.0. We should go through and call all the new methods instead of the old ones.

I suggest we add a private helper module for handling diffpy.structure where we import the correct functions based on the version. I've done this for RosettaSciIO imports in kikuchipy: https://github.com/pyxem/kikuchipy/blob/develop/src/kikuchipy/_utils/rosettasciio_utils.py.

[argerlt]

Why not just set the minimum supported version of diffpy.structure to 3.4 right now on the develop branch, then fix the warnings before 1.7?

I'm down either way, but looking at the warnings, it's lots of remaned Class functions, as opposed to a nice neat module move like with the rosettasciio example.

[hakonanes]

Unless it's too much hassle, I'd like for us to support older versions if we can. Let's see.