Efficiency: setup parallelisation-test for BexBT

[Claudius-Appel]

See #21

• Script must be documented sufficiently to allow users to re-run it.
• results must be saved automatically, and setup must be agnostic of working directory // instruct user to set wd to script-dir
• test performance for all permutations of
  • 4/8/12/14 cores
  • 32/200 images
  • gc()/no gc()