Doppler Broadening

[HunterBelanger]

I have written an implementation of the Sigma1 algorithm to perform doppler broadening, but it has not yet been added to the library. This must be added, and a new type of cross section should be created, allowing users to supply a temperature argument, and receive the cross section at the desired temperature (so long as that temperature is above the initial temperature).

Here are the general requirments to accomplish this:

• Add the Sigma1 implementation to the library
• Add new methods to EnergyGrid, defining the start point of the URR
• Create a new S1CrossSection class
• Create new Reaction template specialization Reaction<S1CrossSection>, which should be aliased as S1Reaction
• Create new CENeutron template specialization CENeutron<S1CrossSection>, which should be aliased as S1Neutron