Enable freezing the position of certain atoms

[Alexsp32]

Similar to #244. Currently, the only way of freezing certain atoms in a simulation in place (at least that I'm aware of) is by using an ase calculator which outputs 0 forces on the respective atoms. However, this alone doesn't work for methods where random noise is applied, such as Langevin dynamics.

In conjunction with #244 , it might be best to allow specifying frozen atoms when defining a Simulation, which makes the necessary modifications to methods. (ignoring initial velocities from DynamicalDistributions, position/velocity updates, random noise, ...)