Add AMReX_SPACEDIM flag to CMake configure step

[BenWibking]

This adds -DAMReX_SPACEDIM= to the CMake configure step when building tests.

Closes #138.

[zingale]

@ax3l is this change okay with you?

[ax3l]

Thanks, looks good!
We added this manually so far, but this is better 👍

https://github.com/ECP-WarpX/WarpX/blob/24.06/Regression/WarpX-tests.ini#L1857-L1859

[ax3l]

Oh, well, technically in CMake we delegate this to application code (e.g., WarpX_DIMS, ImpactX_DIMS), which then sets the option for AMReX in superbuilds, too.

This PR only sets AMReX_SPACEDIM, which we usually do not user-configure from applications.

Maybe we could add an option how we like to call the CMake dims variable to the general config to abstract this?

[BenWibking]

Oh, well, technically in CMake we delegate this to application code (e.g., WarpX_DIMS, ImpactX_DIMS), which then sets the option for AMReX in superbuilds, too.

This PR only sets AMReX_SPACEDIM, which we usually do not user-configure from applications.

In Quokka, we set AMReX_SPACEDIM directly. Is this a reason to prefer setting another app-specific parameter instead, and then set AMReX_SPACEDIM in the app's CMakeList.txt?

[ax3l]

Yes, abstraction. They are different software packages. In practice AMReX can be pre-installed, with 3 different dims at a time. We might just want to build a specific dim or 2 out of three dims of our app.

[BenWibking]

Ah, ok. That makes sense. We have always done superbuilds.

But in any case, having a var to change the name of the dim parameter makes sense to me.

[ax3l]

Oops, should have removed approval. I think we should first add the extra option we mentioned