make clean works quietly and propogates properly
Non-mpi version wasn't compiling

When we implemented dynamic load balancing, we didn't properly use ifdef
directives to cut out the bits that depend on MPI when MPI isn't being used.
Fixing example abc_config_exec.json file

It was looking for an executable in the user's path, and not checking
the working directory.
Adding example programs

Previously examples/main.cpp used an external executable for the simulator.
Now there's an example of how to use a pointer to a simulator function that
doesn't depend on MPI, and main.cpp has been renamed main_exec.cpp. Makefile
has been tweaked accordingly.