Re-arranged the time-step sequence. Still some debugging checks in place.

examples/ExternalPointMass/energy_plot.py

examples/ExternalPointMass/externalPointMass.yml

examples/ExternalPointMass/makeIC.py

src/active.h

src/cell.c

src/cell.h

src/const.h

src/drift.h

src/engine.c

src/engine.h

src/kick.h

src/potential/point_mass/potential.h

src/runner.c

src/space.c