EPPT-2721 contrails output categorical data

[ryan-cocking-mo]

Ticket: https://metoffice.atlassian.net/browse/EPPT-2721

• Output categorical cube from contrail class
• Error checking of input cubes and arrays
• Unit tests
• Refactoring of contrails class

[codecov]

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[ryan-cocking-mo]

Rebased onto EPPT_2072_contrails_design following local vapour pressure calculation fix from ticket: https://metoffice.atlassian.net/browse/EPPT-2784

[mo-robertneal]

The CondensationTrailFormation class runs as expected and the output looks sensible. I plotted the formation cube for a few pressure levels and found a healthy number of 0s, 1s, and 2s. The unit tests all pass and the tests look sensible to me. The only to change suggestions I have are:

1. Do we need to keep the inherited air_temperature status_flag within the auxiliary coordinate? If not, this could be deleted in the _create_contrail_formation_cube function.
2. Change 273.15 on L292 to a constant imported from improver.constants.

[ryan-cocking-mo]

The CondensationTrailFormation class runs as expected and the output looks sensible. I plotted the formation cube for a few pressure levels and found a healthy number of 0s, 1s, and 2s. The unit tests all pass and the tests look sensible to me. The only to change suggestions I have are:

1. Do we need to keep the inherited `air_temperature status_flag` within the auxiliary coordinate? If not, this could be deleted in the `_create_contrail_formation_cube` function.

2. Change 273.15 on L292 to a constant imported from `improver.constants`.

I have applied change 2. As for change 1, I do not think this is necessary, as epp_workflows already removes this flag in the temperature_on_pressure_levels diagnostic. See here (an identical diagnostic is present in eppgl_contrails).

[mo-robertneal]

Changes look good to me. Happy to approve.

[Katie-Howard]

I've looked through the code changes and they all look fine to me - there are unit tests which are sensible and they all pass. So I'm happy to approve 👍