Gravity multi dt

Some improvements to the gravity code:

 - Corrected typo in 4th order vector powers.
 - Use a user-defined opening angle for the distance checks.
 - Increase accuracy of the exact gravity calculation.
 - Only do exact gravity calculation if a matching result file is not present in the same directory.
 - Reinstate the dumping of individual task timers.
 - Added script to plot the accuracy of the calculation.
 - Crash if the M2L kernel is called with a length smaller than the softening length.

See merge request !331