Changing text of landing page for DAGMC website.
Add link to CNERG
Small formatting tweaks
Finish cleaning up MeshTally code and added more detail to comments
Adding TallyEvent class to represent abstract physics data interface
Replacing original KDE tally parameters with TallyEvent objects
Replacing original TrackLengthMeshTally parameters with TallyEvent objects
Moved mcnp_weight_calculation calls to meshtal_funcs.cpp
Added a pure virtual compute_score function to MeshTally

* Combined tracklen and kde_track into one track_tallies array