Refactor the functions in derive_evspsblpot due to new iris

[SarahAlidoost]

Description

• Closes Broken hydrology recipes due to iris.exceptions.NotYetImplementedError: Coord AuxCoord #2021
• Link to documentation:

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[SarahAlidoost]

@ESMValGroup/tech-reviewers this PR is ready for review. Who has time for it? Thank you.

[Peter9192]

Hey Sarah, nice digging and I guess the workaround is fine for now. However, I think this doesn't improve the overall clarity of the code (ideally the equations can be mapped on to one to those in the paper), so we should consider reverting this once the bug is properly solved in Iris.

I'm surprised that codacy doesn't complain about the import order. Can you verify with isort?

[Peter9192]

Doesn't isort want it the other way around?

[SarahAlidoost]

Thank you. Running prospector on this file didn't complain about it, pylint says: third party import "import numpy as np" should be placed before "import iris"pylint.

[SarahAlidoost]

@ESMValGroup/esmvaltool-coreteam this PR is ready for a merge. Thank you.