freud 2.13.0 Released

[Tommy Waltmann]

Version 2.13.0 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/

This release adds neighborlist filters and removes support for python 3.6.

### Added
* Filter neighborlists with `freud.locality.FilterSANN` and `freud.locality.FilterRAD`.

### Fixed
* Neighborlists generated by certain compute objects now exist after the compute object is garbage collected.
* All source files have the freud license header at the top.
* The `compute()` method of each compute object returns self.

### Changed
* Documentation renders with the furo theme.

### Removed
* Support for Python 3.6