Add check for the not-yet-implemented functionality of selecting variables from different coord levels in the same input dataset

[ealerskans]

Describe your changes

The current implementation of selection of variables by coordinate values does not, within one input dataset, support the selection of variables from different coordinate levels. E.g. the following selection

    variables:
      u:
        altitude:
          values: [100, 50, ]
          units: m
      v:
        altitude:
          values: [100, 75, ]
          units: m
      t:
        altitude:
          values: [30, ]
          units: m

would currently add u100m, u50m, v100m, v50m, t100m, t50m to the output dataset. So it results in missing variables ( v75m and t30m) but also adds extra variables (v50m, t100m and t50m), which all are just nans. Such a selection is something we would like to support but the current implementation does not.

Implementing the support of such a selection would require changes to the way that the selected (and derived) variables are collected (currently in one dataset before merging and dimension mapping). Until such a time, this PR adds a NotImplementedError exception in validate_config if one tries to do a selection of variables on different coordinate values within one input dataset. It also adds a section to the README with an explanation of the allowed selections and how to do the selection of variables on different coordinate values.

I have also added tests to check that the exceptions raised in validate_config are correctly caught.

No change in dependencies needed for this change.

Issue Link

Addresses #61.

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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