Added external potential energy calculation to statistics.c

examples/main.c

src/Makefile.am

src/cell.c

src/cell.h

src/engine.c

src/potential/disc_patch/potential.h

src/potential/isothermal/potential.h

src/potential/none/potential.h

src/potential/point_mass/potential.h

src/potential/softened_isothermal/potential.h

src/runner.c

src/scheduler.c

src/scheduler.h

src/space.c

src/statistics.c

src/statistics.h

src/threadpool.c