Added tests and documentation for cases zero neighbor cases.

[klywang]

We checked the behavior of Hexatic, Translational, SolidLiquid, and LocalDescriptors handle cases with zero neighbors.

For particles without neighbors:

• Steinhardt: Returns nan for wl=True and wl=False
• Hexatic: Returns nan
• Translational: Returns 0
• SolidLiquid: particle_harmonics returns nan
• LocalDescriptors: num_sphs returns 0, sph=[]

Description

Added tests for edge cases and updated docs where nan is expected.

TODO: Had some issue building the docs. Most of the notes did not show up.

Motivation and Context

Resolves: #678

How Has This Been Tested?

Added unit tests.

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.

[codecov]

Codecov Report

Merging #775 (3c152ae) into master (b960cab) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #775      +/-   ##
==========================================
+ Coverage   55.38%   55.48%   +0.09%     
==========================================
  Files          16       16              
  Lines        2580     2581       +1     
  Branches       38       38              
==========================================
+ Hits         1429     1432       +3     
+ Misses       1135     1133       -2     
  Partials       16       16              

Impacted Files	Coverage Δ
freud/order.pyx	45.91% <100.00%> (+0.86%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b960cab...3c152ae. Read the comment docs.

[klywang]

Force this to be nan by dividing by zero. Is there a better way to do this? I saw std::nanf but I wasn't sure what to pick as the argument.

[vyasr]

I haven't reviewed this PR thoroughly yet, but I'm not sure this is generating the output you want. std::complex<float>(1)/std::complex<float>(0) will return (inf, nan), not just nan (here's an example)

[klywang]

So tried to set m_psy_array equal to just nan like this:

m_psi_array[i] = std::complex<float>(std::nanf)

and then like this

m_psi_array[i] = std::complex<float>(std::nanf)

All tests pass on my laptop, but fail on CI, so I reverted back to a divide by zero. I double checked the values from the no neighbor test for the Translational order parameter, and in python, it registers as nan and not inf.

[vyasr]

@klywang any updates on this?