Temporary PyCIFRW dependency

[hakonanes]

We must add PyCIFRW as a dependency until it again comes automatically by installing diffpy.structure from PyPI. See diffpy/diffpy.structure#97. Will do this for the upcoming v0.13.0.

[hakonanes]

The diffpy guys fixed this in:

• Add dependency install section in pyproject.toml Billingegroup/cookiecutter#110
• Add dependency installation in pyproject.toml for pip install Billingegroup/bg-mpl-stylesheets#60

We can get rid of the explicit PyCIFRW dependency once a new patch v3.2.1 for diffpy.structure is released :)

[hakonanes]

Only diffpy.structure v3.1 is available from conda-forge, which means we get all necessary dependencies as well when running conda install diffpy.structure -c conda-forge. But just for good measure, until we can restrict diffpy.structure>=3.2.1 on conda-forge, I'll list PyCIFRW there as well.