fixes cRK4
This fixes cRK4. The attached file shows that particle trajectories do indeed converge with 4th order when using appropriate interpolation. particle_evdt_all.pdf
Note: the field must be stationary for this plot to be reproduced (or we need the appropriate temporal interpolation to be implemented). With linear interpolation in time, the accuracy is limited to 2nd order.
Edited by Cristian Lalescu