Update DSMC reaction weight calculation

[roelof-groenewald]

This PR updates the DSMC scheme to use the updated reaction weight calculation described by @JustinRayAngus in #5047.

Still to do:

• rerun benchmark calculations to confirm implementation accuracy
• reset checksum values
• add input parameter that sets minimum weight ratio for which particles will be split and recombine particles from the higher weight particle if that value is not exceeded (do this in DSMC: add minimum reaction weight for particle splitting #4976 after this PR is merged)

[roelof-groenewald]

I reran the physics benchmark used to originally validate the DSMC implementation (comparing against the Turner 2013 MCC benchmark). The plots below show the results for the 1st case using different numbers of kinetic neutral particles:

The results match expectation unless the number of neutral particles is too low (neutral density is much higher than plasma density in this problem so good statistics would require more neutral macro-particles than for ions).

[roelof-groenewald]

Here are the results of running the benchmark simulations for cases 2 & 3 as well. The results match expectation in all cases.

[JustinRayAngus]

This all looks good to me.