Feature/standardize query args

[vyasr]

Description

This PR cleans up some of the C++ code for the QueryArgs class, introducing more constants into the namespace and improving code consistency. Query argument names have been standardized further, and updated throughout freud. In particular, most references to rmax, nn, num_neigh, etc have been removed in favor of the standard names r_max and num_neighbors. On the Python side, more of the QueryArgs logic has been standardized by introducing a PairCompute subclass of Compute that encapsulates the standard code that goes at the beginning of every compute or accumulate call. In the process, I've also defined a standard method for classes to expose a set of default query arguments.

Motivation and Context

This PR is necessary to standardize the usage of query arguments (especially default query arguments) throughout freud. It's also consistent with #179 and related changes for improving the API.

How Has This Been Tested?

No new tests have been added yet, they will need to be added once the code is closer to a final state. The main tests we will need to add are ensuring that default query arguments work as expected, and that invalid QueryArgs objects are properly rejected.

Screenshots (if appropriate):

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)

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 Changelog.

[bdice]

Very helpful! @vyasr you said this PR would be hard to review, but I think it went smoothly. 😄
Some suggestions for edits below:

[codecov]

Codecov Report

Merging #395 into next will decrease coverage by 0.34%.
The diff coverage is 83.49%.

@@            Coverage Diff             @@
##             next     #395      +/-   ##
==========================================
- Coverage   93.14%   92.79%   -0.35%     
==========================================
  Files          15       15              
  Lines        3034     3027       -7     
  Branches       19       19              
==========================================
- Hits         2826     2809      -17     
- Misses        200      210      +10     
  Partials        8        8

Impacted Files	Coverage Δ
freud/order.pyx	97.78% <100%> (ø)	⬆️
freud/locality.pxd	92% <100%> (ø)	⬆️
freud/density.pyx	97.95% <100%> (-0.08%)	⬇️
freud/locality.pyx	94.16% <66.66%> (-0.67%)	⬇️
freud/common.pyx	89.69% <70.83%> (-6.2%)	⬇️
freud/environment.pyx	88.93% <92.59%> (-0.08%)	⬇️

---

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 beaca81...e15dab0. Read the comment docs.

[vyasr]

I will draft relevant documentation and add to #348 soon.

[bdice]

Great work! I approve this.