Move mdrun signalling code

Move mdrun signalling code

No functionality changes here, I just put some related functionality
into functions in their own source file to
* simplify later removing inter-simulation signalling, and
* prepare for further encapsulation later.

Updated struct name and usage for new style. Updated include guards.
Removed now-useless C++-compilation guard from header file. Added
basic Doxygen.

Change-Id: I3ea1e2f77a1cdb6608bba38f08a430cc3b8d7c9a