One major change to replace all gpart id/parts with the new id_or_neg_offset.

As discussed in #123 (closed), this merge request changes the gpart.id and gpart.part anonymous union into a single long long int id_or_neg_offset.

If this number is positive, then it is the particle ID and the gpart has no hydro counterpart.

If this number is negative, then it is the offset in the parts array of this particle's hydro counterpart.

examples/main.c

@@ -354,7 +354,7 @@ int main(int argc, char *argv[]) {
     free(parts);
     parts = NULL;
     for (size_t k = 0; k < Ngpart; ++k)
-      if (gparts[k].id > 0) error("Linking problem");
+      if (gparts[k].id_or_neg_offset < 0) error("Linking problem");
     Ngas = 0;
   }