utility bill percent savings calculation fix

[JanghyunJK]

Pull request overview

• extension of this past PR: percentage savings calculation fix in post-processing #271

• pretty much trying to do the same for the new utility bill savings calculation that happened after that previous PR

• before this PR - datapoints representing 'fraction' although axis title is in %:
  

• after this PR - datapoints representing 'percentage':
  

• checked the other figures and didn't find any side effect causing changes in other figures

• but thanks to @ChristopherCaradonna @asparke2 for conceptually fixing this already 🤟

Pull Request Author

This pull request makes changes to (select all the apply):

• Documentation
• Infrastructure (includes apptainer image, buildstock batch, dependencies, continuous integration tests)
• Sampling
• Workflow Measures
• Upgrade Measures
• Reporting Measures
• Postprocessing

Author pull request checklist:

• Tagged the pull request with the appropriate label (documentation, infrastructure, sampling, workflow measure, upgrade measure, reporting measure, postprocessing) to help categorize changes in the release notes.
• Added tests for new measures
• Updated measure .xml(s)
• Register values added to comstock_column_definitions.csv
• Both options_lookup.tsv files updated
• 10k+ test run
• Change documentation written
• Measure documentation written
• ComStock documentation updated
• Changes reflected in example .yml files
• Changes reflected in README.md files
• Added 'See ComStock License' language to first two lines of each code file
• Implements corresponding measure tests and indexing path in test/reporting_measure_tests.txt, test/workflow_measure_tests.txt, or test/upgrade_measure_tests.txt
• All new and existing tests pass the CI

Review Checklist

This will not be exhaustively relevant to every PR.

• Perform a code review on GitHub
• All related changes have been implemented: data and method additions, changes, tests
• If fixing a defect, verify by running develop branch and reproducing defect, then running PR and reproducing fix
• Reviewed change documentation
• Ensured code files contain License reference
• Results differences are reasonable
• Make sure the newly added measures has been added with tests and indexed properly
• CI status: all tests pass