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

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

Cristian Lalescu authored Jul 05, 2020 and Cristian Lalescu committed Jul 22, 2020

not checked for correctness, just checked that code runs correctly.

particle file layout is read from input by particle set, and then the
`particle_solver` object gets it from there when it needs it, so:
 - user-facing code is less verbose.
 - generic operating mode is implemented only once.

broken, because I'm doing something wrong with the arrays pased to
p2p_distr_mpi::compute_distr.

preliminary. in particular solvers cannot be a simple function, they
should handle checkpointing --- especially if Adams-Bashforth particles
are to be reimplemented with the new classes.