RDF Normalization Mode Argument

[tommy-waltmann]

Description

This PR changes the normalize argument to freud.density.RDF to be more descriptive, and clarifies what the default value is.

Motivation and Context

There was a lot of discussion about how the RDF should be normalized, what the options should be, and how the API should look in the issues linked below.

Resolves: #635
Related to #396

How Has This Been Tested?

I have added a test for both of the valid arguments, and a test case for the invalid arguments.

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.

[DomFijan]

I can take a look at this on Friday.

[DomFijan]

This looks good. I just two questions related to docstrings.

[vyasr]

I've updated the PR description to say that this is related to #396 but not that it resolves it. There is additional work to be done to resolve that issue.

[tommy-waltmann]

I've updated the PR description to say that this is related to #396 but not that it resolves it. There is additional work to be done to resolve that issue.

Good point, I'm not currently planning on implementing the new solution on this PR, so it wouldn't resolve that issue.

[DomFijan]

I think there is a typo in one of the changes to docstrings (please double check that). And I have some improvements suggested. I can't see anything else wrong.

[vyasr]

It would be nice to actually validate the long-distance behaviors of the RDF for a small system of particles as well, but getting that test to behave nicely statistically will probably be hard since the whole point is to test a small system where statistics are bad, so if you have a hard time writing a robust test it's fine to skip it.