Infrastructure for regridding and restarting Neverworld2 output at higher resolution

[glwagner]

In order to run a hierarchy of Neverworld2 simulations, we need to be able to regrid the state from a Neverworld2 simulation to a higher resolution.

For this we can use regrid!, and possibly also diffuse_tracers! to smooth fields out. We may also need continue_downwards! (hopefully we don't need horizontal inpainting?)

cc @simone-silvestri @sandreza @navidcy