Corbett/references and exceptions

[corbett5]

The first commit changes almost every function in Group and Wrapper to return / except references instead of pointers. If you look at the changes to those files there shouldn't be too much need to look at the other hundreds of files because the changes there follow directly from Group and Wrapper.

The second commit adds exceptions to key paths utilized by the python interface and adds error handling in the interface itself. Ideally the goal should be that as long as the Python user doesn't directly modify the data repository (ie completely muck up the mesh maps for example) they should not be able to crash the program. This doesn't achieve that, since I'm sure they could feed in a bunk XML file that would trigger some sort of GEOSX_ERROR that I didn't change to a GEOSX_THROW but it goes a long way in protecting them from bad command line arguments and paths to get_group and get_wrapper.

Related to
GEOS-DEV/LvArray#224
GEOS-DEV/GEOSX_PTP#24
https://github.com/GEOSX/integratedTests/pull/118

[corbett5]

Passing all but the submodule check.