particle system needs vorticity field, then does its own interpolation.

not tested yet.

code runs without segfaults. I still need to test the correctness.

very simple functionality, but I gain two lines per value in the useful
file, which I consider a big deal.

still a work in progress because the code will only work with the Euler
integration scheme for now.
we need to switch `enforce_unit_orientation` to work on the orientation
itself, and to call it in a different place in the code.

This code fails for particles with interactions.
Most likely because the time step is too large.
But we need the commit

We will never use AdamsBashforth with more than 10 steps.