Use more macros

[ElDeveloper]

In #18 and maybe elswhere there's a few patterns that could be abstracted into a few macros, for example

if(obs_indices_resident == NULL) {
    fprintf(stderr, "Failed to allocate %zd bytes; [%s]:%d\n", 
            sizeof(uint32_t**) * n_obs, __FILE__, __LINE__);
    exit(EXIT_FAILURE);
}

To something like:

#define fail_if_null(mem, size) {\
 if (mem == NULL) { \
     fprintf(stderr, "Failed to allocate %zd bytes; [%s]:%d\n",\
             size, __FILE__, __LINE__);\
     exit(EXIT_FAILURE);\
 } \
}

So all these lines turn into:

fail_if_null(obs_indices_resident, sizeof(uint32_t**) * n_obs);

disclaimer: my C is a bit rusty and I haven't checked that the code above compiles.