Add cross-collisions diagnostics to the GK LBO app

[Maxwell-Rosen]

Feature

This PR is nearly 100% Claude, but it's not AI slop. I verify that this implementation is how I would do it, also.

Summary

Purpose: My mirror simulations are exposing an issue with the implementation of cross-species collisions. The moments can go negative if $m_i \gg m_e$, $T_e \ll T_i$, $u_{\parallel,i} \ll u_{\parallel,e}$. This is just in the mathematics of how these cross-moments are computed. By outputting detailed cross-species collision diagnostics, we can effectively diagnose such issues.

Implementation Details

Key changes: gk_species_lbo.c is expanded to include a new diagnostic for the cross-species collisions. Since these diagnostics are on intermediate quantities, they must be re-computed, so I refactored the update function as well.

I decided to reuse some memory from the prim_moms_ho and nu_sum_ho to save some memory and avoid additional allocations.

I could see someone arguing that this should just be under the write_diagnostics flag and not a separate flag. It's good to keep this modular.

Automated testing: Existing tests

Example Use

    .collisions =  {
      .collision_id = GKYL_LBO_COLLISIONS,
      .den_ref = ctx.n0,
      .temp_ref = ctx.Ti0,
      .num_cross_collisions = 1,
      .collide_with = { "elc" },
      .write_diagnostics = true,
    },

Community Standards

• Documentation has been updated.
• My code follows the project's coding guidelines.
• Changes to layer/zero should have a unit test, e.g., core/zero.

Testing:

• I added a regression test to test this feature.
• I added this feature to an existing regression test.
• I added a unit test to test this feature.
• Ran make check and unit tests all pass.
• I ran the code with make valcheck, and it is clean.
• I ran the code through computer-sanitizer on GPU, and it is clean.
• I ran a few regression tests to ensure no apparent errors.
• Tested and works on CPU.
• Tested and works on multi-CPU.
• Tested and works on GPU.
• Tested and works on multi-GPU.

Additional Notes

[Maxwell-Rosen]

Continued in #1076