Fix mistake in the sink loops

The current sink code is incorrect:

• symmetric calls were not made
• the symmetric function is empty in the GEAR version.

The loops are not treated in a symmetric way. By convention iact(a,b) has to yield the same result as iact_non_sym(a,b); iact_non_sym(b,a);.

I have fixed the code in src/runner_doiact_functions_hydro.h to make sure the symmetric calls are made. But there is still a mistake in the GEAR implementation as the symmetric function is empty.