Fix energy bin sampling in source
Created by: brbass
The algorithm to find the appropriate energy group for a sampled particle in source.F90 assumes the energy bounds are ordered from lowest to highest. The current implementation reverses the energy bins to be from highest to lowest in input_xml.F90.
This change reverts the energy groups to their original order, from lowest to highest. This has a side effect of aligning the energy bins with the tally bins, which previously were reversed.
After the change, test_mgxs_library_ce_to_mg.py fails. It looks like the the difference is probably due to a change in order of some random operation on the flipped array of energy bins. As I increase the number of particles, the old and new results converge to the same eigenvalue and same cross sections. I went ahead and updated the results of the test for now.