freud 2.9.0 Released

[Tommy Waltmann]

Version 2.9.0 of freud is now released. Thank you to all who contributed code and reported issues. Please report issues and suggest feature enhancements via the freud GitHub issue tracker.

• conda-forge package: https://anaconda.org/conda-forge/freud
• PyPI package: https://pypi.org/project/freud-analysis/
• GitHub repository: https://github.com/glotzerlab/freud
  
• Documentation: https://freud.readthedocs.io/en/stable/
• Source tarball: https://glotzerlab.engin.umich.edu/Downloads/freud/

### Added
* (breaking) Some `freud.diffraction.StaticStructureFactorDebye` property names changed to be more descriptive.
* `freud.diffraction.DiffractionPattern` now raises an exception when used with non-cubic boxes.

### Fixed
* `freud.diffraction.StaticStructureFactorDebye` implementation now gives `S_k[0] = N`.
* Cython is no longer listed as an install requirement in `setup.py`.

### Removed
* Custom CMake build type `ReleaseWithDocs`.