core: Removed MEMBRANE_COLLISION + out_direction

[fweik]

Fixes #2214. Fixes #3416.

Description of changes:

• Removed membrane collision
• Removed out_direction

[codecov]

Codecov Report

Merging #3418 into python will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           python   #3418   +/-   ##
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  Coverage      86%     86%           
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  Files         534     534           
  Lines       25227   25227           
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  Hits        21793   21793           
  Misses       3434    3434

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[jngrad]

Skimmed through the diff and the changes looked good to me. Can RBCs now cross each other?

[fweik]

Can RBCs now cross each other?

Probably. But I'm not sure what to do about this.

[jngrad]

@fweik actually, the simulation crashes as soon as a RBC makes contact with a shape:

ERROR: Constraint violated by particle 3 dist -0.999825
Traceback (most recent call last):
  File "motivation.py", line 151, in <module>
    system.integrator.run(steps=2500)
  File "integrate.pyx", line 60, in espressomd.integrate.IntegratorHandle.run
  File "integrate.pyx", line 196, in espressomd.integrate.Integrator.run
  File "utils.pyx", line 264, in espressomd.utils.handle_errors
Exception: Encountered errors during integrate: ERROR: Constraint violated by particle 3 dist -0.999825

The bug was present before this PR. We might as well update the espresso features table to mark OIF as experimental. The OIF Sphinx documentation was written as a walkthrough for the sample, so deleting the sample would make the docs difficult to follow.

[fweik]

@jngrad This is potentially because the interaction with the wall is soft. But I see no point in fixing any of this. For me at least, this is far out of scope.

[jngrad]

the interaction with the wall is soft

I used to be able to squeeze RBC through narrow geometries in the past, and they deformed without causing a crash. A regression was introduced at some point, but I won't look into it. This can be merged as is. Even though CI is red, kodiak won't mind and will pull in the pint fix during merging.