Convert power module to xarray

[akeeste]

power.quality

• [x] for functions where pandas is an input, allow xarray as an input
• [x] for functions that take pandas or xarray input, convert the internal functionality to be xarray-based
• [x] for functions that now allow xarray input, add a test
• [x] for functions that output pandas, add a flag to output xarray

power.characteristics

• [x] for functions where pandas is an input, allow xarray as an input
• [x] for functions that take pandas or xarray input, convert the internal functionality to be xarray-based
• [x] for functions that now allow xarray input, add a test
• [x] for functions that output pandas, add a flag to output xarray

TODO

• [x] confirm that example notebooks using the power module still run correctly
• [x] update "dimension" arguments to "time_dimension" or "frequency_dimension" as noted in #279

[akeeste]

@ssolson Thanks for the review. I went through your comments and resolved them, which were primarily on adding parameter validation and doc strings where missing. I reverted the test to recalculate the answer as described above, and double-checked that the power notebook runs. I think the only outstanding item is to ensure that the THCD calculation is not missing anything.