## sample target energy directly!

*Created by: walshjon*

This PR changes the accelerated resonance elastic scattering algorithm based on the double-integral change of variables (`mu`

, `v_t`

--> `E_rel`

, `E_t`

) so the `E_rel`

**and** `E_t`

are now both sampled directly and there is a single rejection criterion (that the resulting `mu`

must be on [-1, 1]). Upscatter probabilities, scattering kernels, and k_eff values look very good using DBRC as a reference (the two methods should and do produce statistically indistinguishable results).

In preliminary testing with a pin cell and a single resonant isotope (U-238) I've observed this to be faster than DBRC, somewhat faster than ares, and within 1% of the standard constant cross section free gas model (`cxs`

). More isotopes should compound this.