Res scat

This pull request enables the exact treatment of resonance elastic scattering through implementations of the Doppler broadening rejection correction (DBRC) and the weight correction method (WCM).  Differential and integral verifications of these implementations were performed by comparing scattering kernels and eigenvalues, respectively, to literature results.  The free gas-constant cross section scattering model, which has historically been used by Monte Carlo codes to sample target velocities, is retained as the default treatment.  If an exact resonance scattering treatment, which accounts for the energy dependence of cross sections, is desired, the user should include an element similar to the one given below in the ```settings.xml``` file.  Users will need to generate 0K elastic scattering cross section data for any nuclide requiring an exact resonance elastic scattering treatment.  Permitted ```method``` inputs are ```dbrc```, ```wcm```, and ```cxs```.  The ```cxs``` method is nothing more than the default free gas-constant cross section scattering model mentioned previously.  However, including it within the ```resonance_scattering``` element allows the user to extend or restrict its application to custom energy ranges.  The default range is cut off at 400kT.  Below ```E_min```, the ```cxs``` default method is applied and above ```E_max```, the target-at-rest asymptotic kernel is used.  An arbitrary number of ```scatterer``` elements may be specified, each corresponding to a single nuclide at a single material temperature.  As indicated below, the implementation of an accelerated target velocity sampling scheme is planned, along with appropriate modification to OpenMC documentation.

```xml
    <resonance_scattering>
      <scatterer>
        <nuclide>U-238</nuclide>
        <method>DBRC</method>
        <xs_label>92238.72c</xs_label>
        <xs_label_0K>92238.00c</xs_label_0K>
        <E_min>5.0e-6</E_min>
        <E_max>40.0e-6</E_max>
      </scatterer>
      <scatterer>
        <nuclide>Pu-239</nuclide>
        <method>wcm</method>
        <xs_label>94239.72c</xs_label>
        <xs_label_0K>94239.00c</xs_label_0K>
        <E_min>5.0e-6</E_min>
        <E_max>210.0e-6</E_max>
      </scatterer>
    </resonance_scattering>
```
- [x] implement the DBRC
- [x] implement the WCM
- [ ] implement the accelerated resonant target sampling scheme
- [ ] document OpenMC resonance scattering treatments