Add tests/notebook/documentation for Utah state income tax logic

[rickecon]

PR #24 adds the Utah state income tax logic to the model. But we still need to do the following

• Add tests for each of the variables in the /fiscalsim_us/variables/gov/states/ut/tax/income/ folder.
• Add a notebook that does the analysis from the Utah Rate Cut Research in Focus (Feb. 2023) article.
• Add documentation regarding Utah.