The yarr implementation compiles but does not seem to update the
momenta. Rather than debug this now (as we have quite a lot of writing
to do) let's delete it and come back to this later.
We are now updating the positions and momenta but the yarr version
does not match the repa version with an unacceptable relative error of
1e-5. We could accept something in the last decimal place (round about
1e-14) but not this much.