Add support for scalar diagnostics

[szy21]

Currently, scalar diagnostics need to be saved as vectors, as the diagnostics need to be mutable. We also don't support writing scalar diagnostics to netcdf, as diagnostics are assumed to be interpolated.

[Sbozzolo]

With #99, we can now write PointSpaces. One easy way (but not super elegant) to support scalars is to create a Field defined on a PointSpace.