Ensure compilation with double precision particles for DSMC collisions

[aveksler1]

DSMC collisions are not properly captured with single precision for typical plasma collision cross sections. This adds an assert to ensure compilation with double precision.

I could imagine a scenario where a user might still want to use single precision for some cross sections that would be captured, so this could we a warning too rather than an assert.

[archermarx]

Is there some way we can scale the cross sections to make them work numerically with single-precision particles?

[roelof-groenewald]

Is there some way we can scale the cross sections to make them work numerically with single-precision particles?

@archermarx we should be able to make single precision work for the collisions. I expect the problem is more likely in the calculation of the collision energy than the cross-sections themselves since I'm fairly certain the MCC routine works for single precision (which uses similar cross-sections). We can open an issue to remind us to sort out this issue in the future since I won't have time this week to look into this deeper.

[ax3l]

LGTM - note: if you do need SP runs of WarpX where DSMC is just a small part, you could promote the DSMC logic to be all double until you have found a way to renormalize.

This would be a bigger change, so erroring out at runtime is fine for me. Please double check the constructor where this is placed does not throw when DSMC is not used.

[roelof-groenewald]

Thanks!

[aveksler1]

LGTM - note: if you do need SP runs of WarpX where DSMC is just a small part, you could promote the DSMC logic to be all double until you have found a way to renormalize.

This would be a bigger change, so erroring out at runtime is fine for me. Please double check the constructor where this is placed does not throw when DSMC is not used.

Just double checked, it's working as intended.