Refactor visualize module into multiple files

[btjanaka]

Description

The visualize module has grown to ~1000 lines, and we expect it to get longer as we introduce further functionality. This length is also making it difficult to run tools like Pylint, which consume more time with file size. Thus, this PR breaks the visualize module into multiple smaller files.

The best way to do this while preserving history is to create multiple branches where we make copies of visualize.py and then merge the copies. This has the drawback of adding many commits to master as we cannot squash the commits (that would destroy the history). For more information on this procedure of splitting files while maintaining history, see here.

In addition to the commits which copy visualize.py, this PR also adds a commit that tidies up the copies by removing the duplicated code.

TODO

• Split methods into separate files
• Tidy up files, including creating _utils
• Check that documentation works
• Split tests into multiple files (alright to not preserve history here)
  • Will NOT do this since we already have visualization

Questions

Status

• I have read the guidelines in
  CONTRIBUTING.md
• I have formatted my code using yapf
• I have tested my code by running pytest
• I have linted my code with pylint
• I have added a one-line description of my change to the changelog in
  HISTORY.md
• This PR is ready to go