RT: update of debugging scheme, added debugging routines to GEAR scheme

for the debugging scheme, I needed to add counts for RT interactions during runner_doiact_functions_hydro loops to avoid early exit where there should be none. I encountered a particle that was correctly found and called, but as it had no neighbours within appropriate distance, it was never marked as treated. Without this mark, the debugging scheme crashes.

Secondly, I added most of the debugging checks and tests also to the GEAR scheme to be run when debugging checks are activated. I added a macro SWIFT_RT_DEBUG_CHECKS which is set in the configure.ac file. The macro is defined either when the scheme --with-rt=debug is chosen, or when --with-rt=GEAR_* and --enable-debugging-checks is set. The GEAR scheme now also passes all my debugging checks and tests.