LRA Bug Fix, Error Messages, and Scattering

I made some bug fixes and slight improvements, including:

- LRA.py bug fixed.  This was a relic of the change from 0-based to 1-based indexing in the getters and setters for group-wise cross sections.
- A large portion of the error messages for getting and setting cross sections had parameters out of order or the wrong number of parameters.  All fixed.
- For group-to-group scattering, all "group1" and "group2" were renamed to more intuitive "origin" and "destination".  A consistent convention of (origin,destination) is used for all routines.
- Inline function for getting the scattering cross section by group was introduced.  This keeps all the logic for how the scattering matrix is stored in one place (at least for getters, maybe the setters could use a similar fix).
- Better error messages are thrown when files sent to materialize don't exist.

I certainly didn't make these routines, so I can't say why they were written this way definitively.  But as I recall, you've told me that the pure array setters are just intended for Python usage.  That said, I don't see why they wouldn't work here.  But do we really care about the performance of clone()?

I'll leave these alone for now, but we can revamp them later if desired.  I need to make some clone routines for my Isotopes and the like at some point, and I can include these in that effort.