Added various fixes for hybrid pressure coordinates

[schlunma]

Added various fixes for CMIP5 and CMIP6 models concerning hybrid pressure coordinates (variables cl, cli and clw).

Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• This pull request has a descriptive title that can be used in a changelog
• Add unit tests
• Public functions should have a numpy-style docstring so they appear properly in the API documentation. For all other functions a one line docstring is sufficient.
• If writing a new/modified preprocessor function, please update the documentation
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Codacy code quality checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.
• Please use yamllint to check that your YAML files do not contain mistakes
• If you make backward incompatible changes to the recipe format, make a new pull request in the ESMValTool repository and add the link below

If you need help with any of the tasks above, please do not hesitate to ask by commenting in the issue or pull request.

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Related to #536, #540, #541, #542. (Note: Automatic closing of these issues is not desired; they should be closed when the issue is reported to the modelers).

[zklaus]

Left some comments, did not review the tests yet.

[schlunma]

@zklaus I rebased the master and added your suggestions.

Please not that I intentionally did not use

from ..common import ClFixHybridPressureCoord as Cl

but

from ..common import ClFixHybridPressureCoord

Cl = ClFixHybridPressureCoord

for two reasons: First, to mute Codacy and second, to make it clearer that there is a fix for Cl in the file, otherwise I think that it might not be so obvious.

[zklaus]

I didn't check all tests in detail, but to me it seems fine.

[bouweandela]

Same here.