dimension independence (avoid needing to set AMReX_SPACEDIM)

[BenWibking]

Describe the proposal
In principle, we can build Quokka in a dimension-independent way, such that we don't need to set AMReX_SPACEDIM.

In practice, this will require setting AMReX_SPACEDIM=3 and then modifying the code to handle the case when Ny==1 and Nz==1 (for 1D sims) or when Nz==1 (for 2D sims) as special cases.

Describe alternatives you've considered
Keep the code as-is (re-build with different values of AMReX_SPACEDIM for different problems).

Additional context
Some new features in AMReX may make the code changes easier: AMReX-Codes/amrex#4016.
See also: AMReX-Codes/amrex#3955