GROMACS

Unlike GROMACS 2020, GROMACS 2021 should have the sd integrator available in modular simulator.

From #2944:

We are planning on implementing the framework presented be Leimkuhler et al. (https://aip.scitation.org/doi/10.1063/1.4802990, also discussed in https://pubs.acs.org/doi/abs/10.1021/jp411770f). Note that this would not just be a SINGLE Langevin integrator, but the framework to swap around orders of the different steps (velocity/position/Ornstein-Uhlenbeck process), so that a number of different algorithms could be implemented easily.

So part of this task is to understand whether we need a new integrator flag, and which scheme the current integrator = sd flag should call.