Decide api for system-wide propagation setup

[RudolfWeeber]

Background

• introduce better control over propagation in ESPResSo
• avoid the currently somewhat arbitrary split of funcitonality between integrators and theremostats. In the end, users need to control the "equation of motion"
• the first part of this was to allow a per-particle override of the system-wide default. This was merged end of 2023.
• now, per-particle selection of propagation is quite different from system wide setup.
• the goal is to have a consistent API for system-wide setup of propagation, i.e., the default that applies to all particles that do not explicitly override it.

Possible API

These are examples for a suggested API for system wide propagation setup, using the same Propagation objects that would be used to control propagation on the particle level.

from espressomd import Propagation

# Langevin for translation and rotation
system.propagation.set_params(
    default=Propagation.TRANS_LANGVEVIN || Propagation.ROT_LANGEVIN,
    kT=1,
    gamma_trans = 1,
    gamma_rot=2)

# LB coupling for translation and Langevin for rotation
system.propagation.set_params(
    default=Propagation.TRANS_LB || Propagation.ROT_LANGEVIN,
    kT=1,
    gamma_trans = 1,
    gamma_rot=2)

# ovverride for an individual particle

system.lb = LBFluidWalberla(...)
lb_tracer = system.part.add(pos=..., propagation=Propagation.LB_TRACER)

system.propagation.run(10)

One drawback is that it is necessary to import the Propagation class in all simulation scripts. On the other hand, then, the API for indidividual particles and the system-default are consistent.