Add neutron inelastic process (#1187)* Add the first skeleton of neutron inelastic model and process, and associated cross section data* Add neutron inelastic model and process* Add ChannelId for neutron codes* Add NeutronXsType* Add missing braces for around initialization of Real3 input data* Change StepanovParameters::xs_o to xs_zero* Change execute with CELER_NOT_IMPLEMENTED* Use GenericGridBuilder* Restructure the NN cross section calculation below 10 MeV* Add an assertion for the positive slope