Version 0.9 testing

[Paul Romano]

Hi OpenMC devs,

Although it took longer than I anticipated, we now have a release-0.9 branch out for testing. To name a few of the significant changes/additions in this release:

- Stochastic volume calculations

- Multi-delayed group cross section generation

- Temperature interpolation on cross section data

- Calculating MGXS over meshes

- Random sphere packing for TRISO particle generation

- Critical eigenvalue search function

- Energy function tally filters

- Replaced xml-fortran parser with pugixml

- Differential tallies

- Nuclear data interface in the Python API

- Consistent multi-group scattering matrices

- OpenMOC compatibility module

- Much improved documentation

I'm going to plan on putting out the release sometime next week. In the meantime, please test this branch and let me know if you run into any major problems while using it. If you find small bugs you think should be fixed before the release, please submit a PR with the release-0.9 branch as the target.

Thanks to all for your contributions; it's amazing to see how far OpenMC has come since it's inception!

Best regards,

Paul