Throw warning if _mesh is not the same for all Fields when creating FieldSet

[erikvansebille]

When creating a FieldSet, we should check that the _mesh of all the grids is the same. It probably doesn't make sense to mix mesh="flat" and mesh="spherical" grids in one simulation

Originally posted by @VeckoTheGecko in #2152 (comment)

[willirath]

It probably doesn't make sense to mix mesh="flat" and mesh="spherical" grids in one simulation

Not entirely sure about that: There might be users which want to provide some fields for only partial domains where they may resort to local projections in meters. So I'd argue for a warning?