Exit make loop on subdir make failure.
Use inline ctor/dtor for Cart3f classes; resolves link-time conflicts.
Add "SimpleMotion" simple motion propagator.
Statically define BUILDROOT before build.
Comment out "const int ZERO". Not currently used.

Leaving it in causes compile fails on some platforms.
Add FlightVehicle class, enable SimpleMotion build.
Add Makefile for command line apps.