Floor cross species thermal velocities at zero

[Maxwell-Rosen]

There is a fundamental flaw with the equations we use to compute the cross-species thermal velocities. These equations are defined in:
M. Francisquez, J. Juno, A. Hakim, G. W. Hammett, and D. R. Ernst, Improved multispecies Dougherty collisions, J. Plasma Phys. 88, 905880303 (2022).

The key equations are 2.9 and 2.10

When the mass ratio is large enough and Ti >> Te, this can lead to a negative thermal velocity squared.

To patch this, we insert the appropriate lines of code after the cross species thermal velocity is calculated

// Floor the cell-average of vtSq_{sr} to zero, setting the rest of the coefficients to zero as well
gkyl_array_set_offset_range(lbo->cross_vtsq, 1.0, cross_prim_moms, 1*app->basis.num_basis, &app->local);
gkyl_array_max_by_cell_per_cell_avg_range(lbo->cross_vtsq, lbo->cross_vtsq_floor, &app->local);
gkyl_array_set_offset_range(cross_prim_moms, 1.0, lbo->cross_vtsq, 1*app->basis.num_basis, **&app->local);

This is an appropriate patch for robustness; however, a future endeavor will need to reformulate cross-species collisions so as not to produce this error. Rosenbluth collisions will be added in the future, so it's not worth redoing the Dougherty collision-operator formulation for the mirrors.

This PR requires a cross-species collisions diagnostic, so this is combined with #1013.

Before:

After:

[manauref]

i would think that flooring it to the minimum representable temperature (rather than 0) would be better.

Also, I think this needs some serious testing, perhaps with:

• a dedicated collisional relaxation test (adapted to be in the appropriate regime)
• a production TCV IWL run (which are relatively cheap and @Antoinehoff has good baselines for).

[Maxwell-Rosen]

Although I understand the physical reasons for setting the floor to the minimum representable temperature, I do not think this will make a significant numerical difference. I'm skeptical that this change would alter the results meaningfully, but we can do it.