Created by: wbinventor

This PR takes a first stab at addressing #1104 by introducing CASMO's pre-defined energy group structures to openmc.mgxs. The group structures are the same as those I used for my thesis and corresponding publications. This feature is implemented as a static EnergyGroups.get_energy_groups(name) class method where name takes on the name of the desired group structure. As an example, this now allows for the following:

casmo_2group = openmc.mgxs.EnergyGroups.get_energy_groups("CASMO-2")
casmo_8group = openmc.mgxs.EnergyGroups.get_energy_groups("CASMO-8")
casmo_70group = openmc.mgxs.EnergyGroups.get_energy_groups("CASMO-70")