Sink iact overhauled to match BHs

Here's my attempt at overhauling the sink interactions such that they match the infrastructure for all the other particle types. I've used the BHs as a model, as right now the sinks only have swallow interactions.

I think I've made things consistent with the BHs, and the code compiles and appears to successfully run the Plummer sphere example simulation with the GEAR model.

Here's what I've changed:

• All self/pair/sub interaction functions have been moved from runner_sinks.h to a new file runner_doiact_functions_sinks.h for consistency with BHs.
• These functions are now declared in runner_doiact_sinks.h by pasting names around in the same way as the BHs are. All functions have been re-named accordingly.
• I'm now importing these functions in runner_main.h and the new file runner_doiact_sinks.c as appropriate, and I added the relevant lines to runner_iact_undef.h.
• I moved the declarations of the swallowing functions (e.g. runner_do_sinks_gas_swallow_self) from runner_doiact_sinks.h to runner.h for consistency with the BHs.

Please let me know if there's anything I've missed or that I should change. I requested a review from @matthieu but @Roduit please do have a look as well! For now I'm going to take this new code layout and try porting in the density interactions on a new branch.