Eliminate xarray->numpy->xarray conversion in lo_l2

[jaredclaypoole]

Closes #3357

• Also had to add mypy error suppression in an unrelated class definition in lo_l2.py, because mypy is set to ignore typing from out-of-module imports

Change Summary

File changes

In imap_processing/lo/l2/lo_l2.py:

• Function _calculate_rates_and_intensities
  • Change return type from dict[str, np.ndarray] to dict[str, xr.DataArray]
  • No need to introduce a newaxis for energy, geometric_factor, gf_low, and gf_high arrays
    • This is because they are named dimensions in xarray, so the broadcasting is handled automatically
  • Change _divide helper to take, operate on, and return DataArrays instead of numpy arrays
• Function _build_map_dataset
  • change variables parameter type to match the output of _calculate_rates_and_intensities
• Class LoSpinAnglePointingSet
  • Added #type: ignore[misc] to prevent a mypy error:
    • Mypy error: imap_processing/lo/l2/lo_l2.py:441: error: Class cannot subclass "PointingSet" (has type "Any") [misc]
    • Apparently this happens because the pyproject.toml's mypy configuration has settings strict = true and follow_imports = skip
      • This means types aren't imported from other modules, so mypy thinks PointingSet (which was defined in another module) has type Any, meaning strict = True prevents it from being subclassed

[jaredclaypoole]

Okay, after bypassing some pre-commit weirdness I no longer have to use a type: ignore[misc] comment. That's good, because it was completely irrelevant to the changes I intended to make.

[tmplummer]

Overall, this looks like a good improvement. I'd like @vineetbansal to look at this too.

[tmplummer]

I think this can be simplified by calling xr.where directly:

Suggested change

	return (numerator / denominator.where(exposed)).where(exposed, 0)
	return xr.where(exposed, numerator / denominator, 0)

[jaredclaypoole]

Good point. And I think that's equivalent to the following, which is more similar to the rest of the code change (calling where on the DataArray directly).

return (numerator / denominator).where(exposed, 0)

I did the above in my latest commit.

[tmplummer]

With the simplification of _divide it seems like this code might be easier to read by directly implementing the division using the where function here. What do you think. IMO, it would make it clearer that it is using the exposed mask. For exampe:

intensity_upper = xr.where(exposed, count_rate / (gf_low.where(valid, 1.0) * energy), 0)

Just a thought.

[jaredclaypoole]

If _divide were used just once, I'd completely agree. But since the same logic gets used 6 times, my preference would be to keep the helper. Feel free to push back.

[jaredclaypoole]

@tmplummer I replied to your comments and pushed another commit. Let me know what you think.

Also added @vineetbansal as a reviewer. (I think he's out until Monday, but we can wait until he's back.)