Refine UA and balance point calculations through outlier detection

[mciethan]

fixes #13 fixes #49 fixes #50

To do:

• [x] add outlier removal functionality
• [x] make sure requirements.txt reflects the addition of numpy to pyproject.toml
• [x] raise some kind of error if balance point goes as high as thermostat set point (meaning we can't produce a valid estimate for that home)
• [x] add a parameter for the minimum reduction in stdev percentage that makes the removal of an outlier worth it
• [x] consider making a BillingPeriod class which bundles together start date, end date, usage, etc
• [x] add some more tests