## fix accelerated resonance scattering bugs identified by @paulromano

*Created by: walshjon*

This PR fixes two bugs identified by @paulromano in the accelerated resonance elastic scattering method:

-- the reduced energy minus `4`

, `E_red - 4`

can sometimes take on negative values at low energies, in which case the lower bound energy should be `ZERO`

-- in computing the maximum value of the Maxwellian target **velocity** distribution as a function of **energy**, I had been using `kT`

, the energy of the most probable velocity, instead of `kT/TWO`

, the most probable energy (which should be used since that is what the sampled Maxwellian is a function of, as is apparent by simply identifying the max of the distribution function) ... This bug has a surprisingly small effect on scattering kernels, upscatter probabilities, and reactor eigenvalues because the double differential PDF for `mu`

and `v_t`

is dominated by 0 K cross sections resonances. In the Doppler Defect Benchmark 5.0% UOX case, for example, DBRC, the bugged ARES, and the de-bugged ARES methods were all separated by 1 pcm with a 1sigma uncertainty of 5 pcm.