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.