Add refractive index modeling to environment

Both radio and Cherenkov need support for evaluating the refractive index at a given particle track location. The key components (as I see it so far):

• Definition of a compile-time interface for all refractive index models - probably something like GetRIndex(Point const&) -> double (since n is unit-less). The implementation should be flexible enough to allow for alternative interfaces for exploring effects like birefringence in the future.
• Implementation of a simple constant index model for validation (and for in-ice showers)
• Implementation of a simple exponential model for comparison with Aires.
• Integration of this interface with the LayeredSphericalAtmosphereBuilder to easily create layered linear+exponential atmospheres.

Feel free to add to this if there are additional requirements at this point.