Improve Recoil Treatment (Conservation instead of gamma = delta)

[nuclearGoblin]

I am submitting a feature request.

The feature I am requesting is for:
realizeCascades (in cascadeProd)

I am requesting:
An improvement on an existing feature. Currently, recoil simulation assumes that the gamma will have energy equal to the energy associated with de-excitation, rather than accounting for conservation laws (and therefore finding the energy where the nuclear momentum is opposite the photon's).

Ideas for implementation:
See code here for calculations based on conservation laws: https://github.com/villano-lab/neutron-capture/blob/master/nc_kinematics.py#L153

Other Information:
This adjustment may be a good time to start implementing #42 as this code is in multiple places. The impact of this assumption seems to be, roughly, 0.5%, looking at the peak of the first excited state for capture on 28Si.