Zero rotation nematic

[AlainKadar]

Description

Raise a warning when a zero vector is in the orientation list passed to freud.order.Nematic.compute

Motivation and Context

Without this change, if user passes zero vector to compute, they may not realize why the director is incorrect.

How Has This Been Tested?

Test was added to ensure that a warning is raised when the zero vector is passed.

Screenshots

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds or improves functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation improvement (updates to user guides, docstrings, or developer docs)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have updated the documentation (if relevant).
• I have added tests that cover my changes (if relevant).
• All new and existing tests passed.
• I have updated the credits.
• I have updated the Changelog.

[tommy-waltmann]

Looks good, great work on this @AlainKadar !