Update our geometry and model interface to be more general/organized

[nickkamp1]

Modify the interaction loading interface to operate similarly to the geometry/flux loading interfaces, i.e. point to a loader .py file in the appropriate resources/CrossSections/[model] directory which handles them model-specific arguments, etc

Also have a separate method for loading detector geometries that accepts either a string referring toa pre-defined experiment or paths to user-defined density/material files

[austinschneider]

#71 will fix at least the first half of this. We still need to add functionality for pointing to user files/directories.