Port over diffractomer from the old cme_utils repo

While Freud does have diffraction modules, I still think it would be useful to port over the "old" [diffractometer](https://bitbucket.org/cmelab/cme_utils/src/master/) code into this repo.

Start by reviewing the old diffractometer code in the linked Bitbucket cme_utils repository and comparing it with the diffraction modules already present in Freud. Identify the functionality that needs to be ported and verify that the port provides equivalent behavior in this repository.

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