Res scat

This is a continuation of PR #232 which adds a capability for more accurately modeling resonance scattering events.  Documentation of the methods and required input has been added, as has a simple test case.
I believe those indices are correct.  They are the bounding indices of `E_low`.  The `xs_cdf` indices are each one less because `xs_cdf` value i actually contains the integrated CDF value up to i+1.