Add simple compile-time way to determine the number of dimensions of a `Space`

The number of spatial dimensions (1-3) of a Space is fully determined by its type.

For example, a `CenterFiniteDifferenceSpace` is guarenteed to have 1 dimension.
```julia
ndims(::CenterFiniteDifferenceSpace) = 1
```
An `ExtrudedFiniteDifferenceSpace` could have 2 or 3, and we can find which one of the two by looking at the type of grid
```julia
ndims(::ExtrudedFiniteDifferenceSpace{G}) where {G <: ExtrudedSpectralElementGrid1D} = 2`
ndims(::ExtrudedFiniteDifferenceSpace{G}) where {G <: ExtrudedSpectralElementGrid2D} = 3`
```

An alternative to this might to return a tuple with `(n_horizontal, n_vertical)` dimensions

This would help with writing code that more easily supports more configurations.

Start by locating the Space types named in the issue, especially CenterFiniteDifferenceSpace and ExtrudedFiniteDifferenceSpace, and inspect how their grid type parameters are defined. Check existing dimension-related APIs and tests before deciding whether the result should be a scalar or a horizontal/vertical tuple; done means the supported Space configurations expose compile-time dimension information with coverage for the listed cases.

Written by the indexing model from the issue text.