Since rhs objects can contain collision counters, it means we need to be
able to update them, they cannot be const.

problem found by gcc 10

Cristian Lalescu authored Aug 14, 2020 and Cristian Lalescu committed Aug 14, 2020

now we actually use an assert to check that finite differences of
positions are equal to sampled velocities.

also adds preliminary step to sanity check for wiggling

the current solution is ugly, I do a lot of copying of temporary arrays,
I believe we can do better.
But at least it works

this shouldn't lead to a big performance increase of symmetrize itself,
but it should enforce thread-local access to field data. to the extent
that it helps...