Fix k-value/k-vector units for non-cubic systems

[andkerr]

Description

This PR continues work started in #777 and others to ensure Freud's diffraction module computes the correct range of k-values and k-vectors for various grid_size, output_size, zoom,view_orientation, and box shapes, and that diffraction peaks are positioned such that their k-vectors satisfy the following relationship for lattice vectors R (from Wikipedia):

Previous commits fixed diffraction results for simple cubic systems. Ideally, future commits will ensure that freud.diffraction.compute() generates correct k-value, k-vectors and diffraction peaks for:

• cubic boxes that have been extended in the direction parallel to the view axis (i.e. producing the same pattern as the corresponding cubic box)
• orthorhombic boxes whose vectors all have different magnitudes
• boxes rotated such that none of their face normal vectors are parallel to the view axis
• boxes with 2+ face normal vectors projecting equally onto the view axis

Some of these will likely involve changing how the compute method's box_matrix_scale_factor is calculated. It is currently assigned to np.max(system.box.to_matrix()), which produces incorrect diffraction patterns in most cases.

Motivation and Context

Resolves: #750

How Has This Been Tested?

N/A. Commits to this PR will include unit tests to verify that diffraction results are correct.

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 #793 (8e3adbc) into master (a0fc869) will not change coverage.
The diff coverage is n/a.

❗ Current head 8e3adbc differs from pull request most recent head 12e016f. Consider uploading reports for the commit 12e016f to get more accurate results

@@           Coverage Diff           @@
##           master     #793   +/-   ##
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  Coverage   55.57%   55.57%           
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  Files          16       16           
  Lines        2438     2438           
  Branches       33       33           
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  Hits         1355     1355           
  Misses       1070     1070           
  Partials       13       13           

Impacted Files	Coverage Δ
freud/diffraction.pyx	71.64% <ø> (ø)

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