debugging, fix to rounding error in calculating delta time from scale factors

...SubgridTests/StellarEvolution/check_continuous_heating.py

src/runner.c

src/stars/EAGLE/stars.h

src/stars/EAGLE/stars_iact.h

tests/star_tolerance_27_normal.dat

tests/star_tolerance_27_perturbed.dat

tests/star_tolerance_27_perturbed_h.dat

tests/star_tolerance_27_perturbed_h2.dat

tests/testFeedback.c

tests/tolerance_125_normal.dat

tests/tolerance_125_perturbed.dat

tests/tolerance_27_normal.dat

tests/tolerance_27_perturbed.dat

tests/tolerance_27_perturbed_h.dat

tests/tolerance_27_perturbed_h2.dat

tests/tolerance_pair_active.dat

tests/tolerance_pair_force_active.dat

tests/tolerance_periodic_BC_normal.dat

tests/tolerance_periodic_BC_perturbed.dat

tests/tolerance_testInteractions.dat