First cut at VESUVIO fitting function. Refs #6316
Add HermitePolynomials helper functions. Refs #6316
Add neutron mass in AMU to physical constants. Refs #6316
NeutronCountRate fit runs without crashing. Refs #6316
Added separate Compton profile functions. Refs #6316

These will replace NCSCountRate as they are more fleixble and they
isolate the code more effectively. It also allows a new profile to be
used without too much effort.
Debugging NCSCountRate Refs #6316.
Correct the computation of Voigt difference. Refs #6316
Use correct theta calculation. Refs #6316