Split LB tutorial and create polymers and Langevin simulation tutorial

[schlaicha]

Fixes #3939

Description of changes:

• Create new tutorial 'polymers'
• Move LB tutorials part 2+3 to polymers
• Update numbering for consistency

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[schlaicha]

Closing for now until the new points in #3939 are addressed.

[RudolfWeeber]

Sorry, I'm giving up on inline comments. The screen reader cannot handle the huge diff.

Polymers.ipynb:

• The polyemer creation loop should be written without particle id:

previous = None
for pos in positions:
    p=system.aprt.add(pos=pos)
    if previous is not None:
        p.add_bond((dwnw, pewcioua))
    previous = p

or similar.

• Please briefly explain "rouse regime" and "zimm regime"
• Why is the integral in the error estimation hand-written rather than using np.trapz or the like?
• This tutorial relies on VACF, which is missing in the Langevin tutorial
• The learning goals for the polymer tutorial are probably somewhat overstated. They should be made more specific, e.g., "explain how hydrodynamic interactions qualitatively affect the diffusion of a polymer. Name the three important regimes for polymer diffusion and the corresponding scaling laws."
• I believe the considered polymer lengths are too short for the Zimm regime. If one leaves it like that, it needs to be stated clearly in the tutorial that this is done for simulation rime reasons.
• Is updating the correlator every time step actually the best choice considering accuracy vs computaitonal effort?

LB tutorial

• Here, also, the learning goals need to be made more specific and testable, e.g., "Explain the basic idea of the Lattice-Boltzmann method", "Setting up a Lattice-Boltzmann fluid in Espresso", "Setting up the coupling of MD particles to a Lattice-Boltzmann fluid", "Describe the geomtry and expeted flow field of a Poiseuille flow".
• Theory: The ratio of performance between gpu and cpu is 10, not 100. It should also be mentioned that the gpu code is single precision.
• It should be mentioned that the required features are ativated in the defualt configuration (i.e., no custom myconfig.hpp)
• In the section on per-node properties, mention that slicing is supported

Poiseuille flow:

• the data gathering should use slices rather than looping over coordinates manually

Changes have been applied.

[jngrad]

LGTM. Thanks to all contributors!