Remove use of mgxs module in MicroXS.from_model

[paulromano]

Description

Currently, the MicroXS.from_model classmethod has a dilute_initial argument that implements behavior similar to what has now been removed in #2568. In order to remove this from MicroXS, the first step is to get rid of its current reliance on openmc.mgxs to calculate microscopic cross sections (having had a look at mgxs, there's not a clear path in my mind for utilizing the multiply_density option there). This PR simply replaces the use of mgxs with manual creation of tallies that set multiply_density = False. The dilute_initial argument is preserved in this PR but will be removed in a future PR.

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)

[yardasol]

Hi @paulromano,

Do you think at some point in the future it would make sense to use the mgxs module if/when we add support for multiple energy groups in MicroXS?

Even if this is the case, I like that we no longer have a bunch of duplicate flux tallies! I'll approve once the tests past!

[paulromano]

@yardasol Even when we add support for multiple energy groups, I don't think mgxs will really give us much. It still has the problem that there's no clear path for utilizing multiply_density=False. In fact with the current setup in this PR, extending to multiple groups will be very straightforward because we are already using an EnergyFilter.

[yardasol]

Good work @paulromano