Mesh-based weight windows

[nglaser3]

Summary of changes

This PR adds mesh-based weight windows to MCDC. In the current state, only spatial weight windows are supported. Additionally, the particle "passes through" a weight window on collision, as opposed to whenever a particles crosses into a new mesh bin.

Weight windows are stored as three separate 1D arrays of floats: lower_weights, target_weights, upper_weights.
To test the additions, I added a few unit tests to unit/transport/technique/weight_windows.py.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Organization and beautification (changes which improve readability and/or accessibility)

Developer Checklist

• I have read the contributing guide.
• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests pass

Associated Issues and PRs

• closes [Feature] Mesh-based weight windows #393
• related to #

Associated Developers

• Dev: @munkm

[nglaser3]

@munkm @ilhamv should I add support for energy weight windows in this PR as well or do a follow up PR for that? Also, the added documentation to the website in this PR is pretty sad, but I was unsure as to whether I should expand the docs around all of the Techniques in this PR or also just make a follow up PR for that?