[Enhancement] - More general flux and XS collapse

[dodu94]

At the moment the code appears to be working only with a limited number of ready-made XS and only for 5 energy bins. The tool functionality could be relatively easily expanded if connected to a package like F4E_ace (still not publicly distributed) which is able to compute a reaction rate given an .ace file, MT channel and desired energy group. This would also eliminate the need for introducing a new XS format and relative parser as it is done at the moment.

[alexvalentine94]

Any group structure can now be passed. Agreed that we should connect to F4E_ace when this becomes available.