Fix Parallel bug caused by uninitialized variable access in gn_glob[]

Because `gn_glob[]` elements were never initialized on runs with certain numbers of processors elements in `gn_glob[i]` would be garbage. (on my machine the error was caused by gn_glob[i]->len_u_aexp being > 0 when the structure was NULL)

This caused the communication between processors for the Noahs_Dove struct to have differing sizes between ProcID 0 and all other processors which when Bcast was causing memory addresses in Noahs_Dove to be assigned the wrong values.

It seems there is an `init_Generalized_Newtonian` function in `mm_as_alloc.c` but it is outdated and was not even used in `gn_alloc`, so this uses the memset method used in other parts of the code.

Running the test suite now.