Preprocessor module to compute and store ACSIS indicators

[valeriupredoi]

Before you start, please read CONTRIBUTING.md.

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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Closes {Link to corresponding issue}

SUMMARY

Compute and store ACSIS indicators as presented to me by @ed-hawkins - this is done now in a nice way via a preprocessor module with minimum effort and input from the user; the way they can invoke it is via an entry in the recipe:

preprocessors:
  acsis:
    acsis_indices:
      moc_file: /home/users/valeriu/esmvaltool_ACSIS_example/moc_transports.nc
      vn_file: /home/users/valeriu/esmvaltool_ACSIS_example/hadslp2r.pp
      moc_variable: moc_mar_hc10
      vn_variable: UM_0_fc8_vn405

note that files can be stored and read from by the code from the auxiliary_data_dir and also the variables can be anything the user wishes and if they are not specified then the defaults are moc_mar_hc10 and UM_0_fc8_vn405; the module computes the currently implemented ACSIS indicators and attaches them to the OBS6 file that were computed with as attributes.

TODO (still)

I need to improve testing and write documentation.

[valeriupredoi]

closing this due to inactivity, but not deleting the branch just in case this will be revived 👻