Move simulator code to repository root

[tstana]

This issue should be addressed after the following have been closed:

• Add OBC sim extensions #5
• Document OBC sim extensions #6

The current location of OBC simulator-related files is a bit hidden. I suggest doing the following operation:

cd $REPO_ROOT
mkdir msp-sim/
mv msp/simulator/* msp-sim/

That is, move the contents of the folder msp/simulator/ into a new folder msp-sim/ under the repository root.

[tstana]

OBC sim. extensions added as a custom CUBES implementation under the proto-cubes-obc repository.

This issue is not relevant anymore.