Add lambda filter for earthkit.data.Field objects

[frazane]

Purpose

Add a first prototype implementation of a "lambda filter", i.e. a filter wrapping arbitrary user-defined functions.

Code changes

Added a new EarthkitFieldLambdaFilter class. Far from being a complete and truly generic feature but can be improved later.

• For now I implemented a filter for functions acting on earthkit.data.Field objects, later we might add similar classes for xarray-based and numpy-based functions
• It only supports the case of single or multiple fields as inputs and a single output field, but could be extended to be more generic (maybe by extending SimpleFilter or creating a new class).
• Replaces/transforms input fields (not intended to add new fields).

Example config:

- earthkitfieldlambda:
    fn: "path.to.function"
    param: ["u","v"]
    args: [10]
    kwargs: {some_option: True}

Extras

Adapted tests to make a single mars request and use numpy.testing module.

[FussyDuck]

All committers have signed the CLA.

[HCookie]

Thank you for your contribution.

The filter looks good, and is well documentated.
I have left a few comments to be addressed,

• Primarily, we use pytest as our test harness, so some of the if __name__ == "__main__" should be removed.