Initialise feedback properties in space_first_init.c

Hi @bvandenbroucke @matthieu ,

Recently I was doing some tests of the updated thermal delay in SNII feedback and was getting strange results. This made me realize that there is a function in feedback.h with the name feedback_first_init_spart(&sp[k], feedback_properties) and this function is never called in the code. I think it should be called in space_first_init.c, like many other functions with similar names and signatures.

The lack of feedback_first_init_spart(&sp[k], feedback_properties) in space_first_init.c did not lead to any problems before because the current version of feedback_first_init_spart(&sp[k], feedback_properties) only sets a certain number of feedback-related fields to zero, which are zero anyway by default (because of the way how structs are initialised, i.e. with zero bits).

Note that this change is only important for the set-ups that read initial conditions containing star particles (i.e. not cosmological runs, for example).