Update FieldSet.gridset to use a list

[VeckoTheGecko]

This PR updates FieldSet.gridset to be a list of BaseGrid instances (i.e., XGrid of UxGrid objects). It also removes FieldSet.get_fields() since we now have the fields dictionary.

Note that to see if grids are the same, this code looks at whether the grid objects point to the same location in memory (this is the default with the ... in ... when working with custom objects) . This results in something like:

grid1 = XGrid(xgcm.Grid(ds))
fieldset.add_field(Field("field1", data, grid1, mesh_type="flat"))
# ^ gridset size increased by 1

grid2 = XGrid(xgcm.Grid(ds))
fieldset.add_field(Field("field2", data, grid2, mesh_type="flat"))
# ^ gridset size increased by 1

This is because calling XGrid twice results in two different XGrid objects in memory despite them having the same arguments. I think that this is fine (I don't really see an usecase where this would limit us) - keen to discuss if someone has a different opinion on this.

• Chose the correct base branch (v4-dev for v4 changes)
• Fixes Remove GridSet class from Public API #1698
• Added tests