cpp -> src

This PR refactors the build system to place all compiled code in a src/ dir rather than a C++.  This has a couple of important side effects.  This first is that it brings in the AHOT code and some of @howland's preliminary work there. The second is that it fixes a bug caused by #489 where MOAB was a required dependency to build, though it should have been an optional one.

The AHOT stuff is still a work in progress by @howland.  Merging this in will help @howland (and me) maintain upstream changes.