Broken hydrology recipes due to iris.exceptions.NotYetImplementedError: Coord AuxCoord

[SarahAlidoost]

With new version of iris, the hydrology recipes recipe_wflow.yml and recipe_marrmot.yml are broken because the script derive_evspsblpot.py returns the error:

Traceback (most recent call last):
  File "/home/sarah/GitHub/ESMValTool/esmvaltool/diag_scripts/hydrology/marrmot.py", line 172, in <module>
    main(config)
  File "/home/sarah/GitHub/ESMValTool/esmvaltool/diag_scripts/hydrology/marrmot.py", line 118, in main
    pet = debruin_pet(
  File "/home/sarah/GitHub/ESMValTool/esmvaltool/diag_scripts/hydrology/derive_evspsblpot.py", line 94, in debruin_pet
    delta_svp = tetens_derivative(tas)
  File "/home/sarah/GitHub/ESMValTool/esmvaltool/diag_scripts/hydrology/derive_evspsblpot.py", line 35, in tetens_derivative
    exponent = iris.analysis.maths.exp(emp_a * tas / (emp_b + tas))
  File "/home/sarah/anaconda3/envs/esmvaltool/lib/python3.8/site-packages/iris/coords.py", line 452, in __add__
    return self.__binary_operator__(other, self._MODE_ADD)
  File "/home/sarah/anaconda3/envs/esmvaltool/lib/python3.8/site-packages/iris/coords.py", line 425, in __binary_operator__
    raise iris.exceptions.NotYetImplementedError(emsg)
iris.exceptions.NotYetImplementedError: Coord + AuxCoord

I explained the bug here SciTools/iris/issues/4000. As a workaround, the script derive_evspsblpot.py needs refactoring.

[SarahAlidoost]

I made a PR refactoring derive_evspsblpot.py . Now, both recipes runs are successful. The differences between time series of pet obtained by this PR [and esmvalcore 2.2.1 and iris 3.0.1] and pet obtained by original script [and esmvalcore 2.0.0 and iris 2.4.0] are negligible, shown in this figure
.