Automatic testing, pct imputed calculations

[mattwigway]

Bad form I know, but there's a few things in this PR. I ran this code last night to generate new data, so I wanted everything together:

• Remove additive smoothing from entropy calculation
• Add fields to peak sensor-day files about how much of day was imputed
• Only run calculate_peaks for the days that are actually in our analysis window
• Add automated tests for peak calculations, entropy calculations, and time period construction
• Correct off-by-one error in peak calculations uncovered by said test
• Set up GitHub Actions to run tests on each commit