Changing text of landing page for DAGMC website.
Add link to CNERG
Small formatting tweaks
Renamed fmesh_card struct and its members to be less MCNP-centric
Removed fmesh_index variable from MeshTallyInput struct
Changed energy_bin_bounds into std::vector from pointer to Fortran array
Cleaned up MeshTally implementation and structure

* Added more detail to comments
* Renamed MeshTally::ebins to MeshTally::num_energy_bins
* Renamed MeshTally::fmesh to MeshTally::input_data
* Added new virtual functions for common mesh tally operations
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
Merge remote-tracking branch 'origin/meshtally_refactor' into meshtally_refactor