Lo: replace EsaCalibration class with an xarray Dataset

[jaredclaypoole]

Closes #3366

Change Summary

Overview

Lo: replace EsaCalibration class with an xarray Dataset

• EsaCalibration was used only in lo_l2 processing

File changes

• imap_processing/lo/l2/lo_l2.py

  • Function _esa_calibration
    • Changed return type: EsaCalibration -> xr.Dataset
    • Construct a dataset with energy dim rather than EsaCalibration namedtuple fields
  • Function _calculate_rates_and_intensities
    • Takes and uses an xr.Dataset rather than EsaCalibration object
  • Function _build_map_dataset
    • Takes and uses an xr.Dataset rather than EsaCalibration object

• imap_processing/lo/constants.py

  • Removed the now-unused EsaCalibration class

Testing

No change to tests. This code change is exercised by various test functions that call lo_l2 in imap_processing/tests/lo/test_lo_l2.py.

[tmplummer]

LGTM. I appreciate the effort to centralize on using xarray objects. I approve but will let Vineet be the gatekeeper by not actually hitting approve.

[vineetbansal]

Thanks @jaredclaypoole, @tmplummer - looks much cleaner now. I see why we'd want to stick to this pattern in the future.