Add support for anemoi-datasets

[sandorkertesz]

This PR adds support for anemoi-datasets.

Major changes

1. New properties/methods added to Field:

• mars_grid for a latlon grid returns [dx,dy] otherwise metadata("gridName")
• mars_area returns [north, west, south, east]
• resolution

     if grid_type in ("reduced_gg", "reduced_rotated_gg"):
         return self.metadata.get("gridName")
  
     if grid_type == "regular_ll":
         x = self.metadata.get("DxInDegrees")
         y = self.metadata.get("DyInDegrees")
         assert x == y, (x, y)
         return x
  
     if grid_type == "lambert":
         x = self.metadata.get("DxInMetres")
         y = self.metadata.get("DyInMetres")
         assert x == y, (x, y)
         return str(x / 1000).replace(".", "p") + "km"
  

• grid_points() returns (lats, lons)
• grid_points_unrotated()
• rotation

     return (
            self.metadata.get("latitudeOfSouthernPoleInDegrees"),
            self.metadata.get("longitudeOfSouthernPoleInDegrees"),
            self.metadata.get("angleOfRotationInDegrees"),
        )
  

2. Added new FieldList type: FieldArray. Name should be reviewed since earthkit-data already has ArrayFieldList

3. Added the FieldCube and Cubelet classes. A version of it already exists in the feature/tensor branch.

4. The "valid_datetime" metadata key now returns a str instead of datetime.datetime