Make preCICE export paths consistent

[MakisH]

I am renaming all directories in<export: tags to precice-exports, as currently suggested by our documentation.

• flow-over-heated-plate:
  • rename in config and README
  • enable exports by default, instead of the user having to explicitly add these (the case is small and short anyway)
  • check the visualization script and update the picture, including OpenFOAM-DuneFem. @BenjaminRodenberg why was that disabled before in the script, while we had it in the picture? Also, there were points used for OpenFOAM-FEniCS, but this is arbitrary in this context and does not originate from the script.
• flow-over-heated-plate-nearest-projection:
  • rename in (commented-out) config and README. I did not enable it, since it is not directly used by any script in this tutorial.
• quickstart:
  • rename in config and README
  • remove from the clean.sh. This is now covered by the clean_precice_logs.

I could not find any other relevant instances.

Closes #417.

[MakisH]

I have no idea why autopep8 complains in this PR, as I did not touch any Python files here and it has not recently failed on develop either.

[uekerman]

Good step