Fixing incorrect usage of std::shared_ptr

We were creating a new shared_ptr over the address of a Galaxy object which was
already owned by a few shared_ptrs. This meant meant that when the new
std::shared_ptr was destroyed the Galaxy underneath was also destroyed,
invalidating the other shared_ptrs, and bringing chaos to shark.

We were using this new shared_ptr only to avoid changing all the rest of the
code that used the -> syntax to access the members of the Galaxy object. The
correct way of doing this, avoiding deleting the underlying object, is of
course to use a raw Galaxy pointer.

Signed-off-by: Claudia Lagos <claudia.lagos@icrar.org>

src/gas_cooling.cpp

@@ -258,7 +258,7 @@ double GasCooling::cooling_rate(Subhalo &subhalo, Galaxy &galaxy, double z, doub
     }
 
     // Define main galaxy, which would accrete the cooled gas if any.
-    GalaxyPtr central_galaxy(&galaxy);
+    Galaxy *central_galaxy = &galaxy;
 
     // If subhalo does not have a main galaxy (which could happen in satellite subhalos), or subhalo does not have a hot halo, return 0.
     if(subhalo.hot_halo_gas.mass <= 0){