note to self we can remove a line after next openmc release

[shimwell]

When reading through the code I noticed this line and thought it would be nice if openmc checked groups were ascending and then we don't have to

openmc_activator/openmc_activator.py

Line 22 in e3b8259

assert(np.all(ebins[:-1] < ebins[1:]))

so I put in this PR to openmc
openmc-dev/openmc#3372 (review)

and now we can remove one line from the activator code. but we might want to hold of an wait for the next release of openmc.

[shimwell]

We can also remove reduce_chain=True argument in the next release of openmc as this has also been automated
openmc-dev/openmc#3377