add checkstyle

[ezrichards]

Closes #287

• working on adding a fork of Google's checkstyle and making it less aggressive
• also adding it on CI to warn on error
• need to add line length IntelliJ stuff in docs: https://stackoverflow.com/questions/24355976/how-to-change-line-width-in-intellij-from-120-character

[read-the-docs-community]

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