Use multiply_density in depletion reaction rate helper classes

[paulromano]

This PR is a follow on to #2539 and updates the depletion system in OpenMC to utilize the new multiply_density option. Since depletion only requires microscopic reaction rates, there's no need to have the tally system multiply by atom density only to divide it out later. With changes here, we now avoid that.

These changes will allow us to get rid of dilute_initial altogether. In fact, I already have that working on a branch but will save it for a separate PR because it does result in test changes. The changes in this PR require no updates to test results.

[drewejohnson]

Just want to make sure I follow the logic

• new tally feature used in depletion does not scale reaction rate tallies by the number density of the target nuclide.
• Depletion backend doesn't really need reaction rates with units atoms/barn-cm, but we do need them to be absolute
• So we need to re-incorporate the volume of the tallied domain (materials) into the tallies

That all checks out to me. I'm happy to merge this through as is, comments are non-blocking

[paulromano]

@drewejohnson Any further comments on this or do you think it's good to merge? btw, thanks for chiming in! Wasn't sure if you were paying attention to the day-to-day OpenMC happenings anymore 😄

[drewejohnson]

Thanks @paulromano! I see things on occasion and try to help where I can! Not so much day to day, maybe month to month 😅