When calculating the particle step in get_effector_tot, we have to
round up (otherwise we might get an extra round in the for-loop in
BKE_effectors_apply for certain cases).

Example from the report:

• 10.000 particles, Effector Amount 3
• was rounding the step down to 3333
• going into the for-loop for 0, 3333, 6666 and 9999 (4 times)
• now rounding the step up to 3334
• going into the for-loop for 0, 3334 and 6668 (3 times as desired)