Use 'unsinged long' rather than 'long' to match GADGET

Former-commit-id: a52ef3edbc60c4b63cfd8eaba46ed7b5c9e2e39b

src/part.h

@@ -37,7 +37,7 @@ struct part {
     float mass;
     
     /* Particle ID. */
-    long id;
+    unsigned long id;
     
     /* Particle position. */
     double x[3];