Agreed upon version of the density vectorization test

Wrote a unified version of the vectorization test to be used as a basis for the tests of @jwillis, @rgb and @alepper

The test also contains a brute-force calculation to check that any approximation made even in the stock DOPAIR1() function is correct.

src/engine.c

@@ -185,8 +185,8 @@ void engine_redistribute(struct engine *e) {
/* Get the new number of parts for this node, be generous in allocating. */
int nr_parts = 0;
for (int k = 0; k < nr_nodes; k++) nr_parts += counts[k * nr_nodes + nodeID];
-  struct part *parts_new;
-  struct xpart *xparts_new, *xparts = s->xparts;
+  struct part *parts_new = NULL;
+  struct xpart *xparts_new = NULL, *xparts = s->xparts;
if (posix_memalign((void **)&parts_new, part_align,
sizeof(struct part) * nr_parts * 1.2) != 0 ||
posix_memalign((void **)&xparts_new, part_align,
@@ -951,8 +951,8 @@ int engine_exchange_strays(struct engine *e, int offset, int *ind, int N) {
message("sent out %i particles, got %i back.", N, count_in);
if (offset + count_in > s->size_parts) {
s->size_parts = (offset + count_in) * 1.05;
-    struct part *parts_new;
-    struct xpart *xparts_new;
+    struct part *parts_new = NULL;
+    struct xpart *xparts_new = NULL;
if (posix_memalign((void **)&parts_new, part_align,
sizeof(struct part) * s->size_parts) != 0 ||
posix_memalign((void **)&xparts_new, part_align,
@@ -2098,8 +2098,8 @@ void engine_split(struct engine *e, int *grid) {
message("Re-allocating parts array from %i to %i.", s->size_parts,
(int)(s->nr_parts * 1.2));
s->size_parts = s->nr_parts * 1.2;
-  struct part *parts_new;
-  struct xpart *xparts_new;
+  struct part *parts_new = NULL;
+  struct xpart *xparts_new = NULL;
if (posix_memalign((void **)&parts_new, part_align,
sizeof(struct part) * s->size_parts) != 0 ||
posix_memalign((void **)&xparts_new, part_align,