[AP][InitialPlacement] Improved Initial Placement

Found that the Initial Placer stage of the AP flow (after APPack, but
before Detailed Placement) was not working as expected. The intention
was that clusters would be placed at their centroid location accordin to
the flat placement, and if that site was illegal or taken it would take
a nearby point instead (falling back on the original initial placer if
nothing can be found).

To achieve this, I was using a method called find_centroid_neighbor
which I thought would return the nearest legal location to the given
location. This was not correct. This method just creates a bounding-box
and tries to find a random point in that box around the given point.
This was causing our AP flow to move clusters WAY farther than they
wanted, which moved them into places other clusters wanted to go. This
was also not exhaustive, so it was often falling back on the original
approach which was putting clusters in practically random locations. All
of this was causing the post-FL placement from the AP flow to actually
have worse quality than the default AP flow!

To resolve this, I wrote the actual method I was intending. It performs
a BFS-style search from the src location to all legal locations and
returns the closest one. By doing this BFS on the compressed grid, I
found that this is actually quite efficient. With these changes, I found
that the quality of the post-FL placement more than doubled and the
average atom displacement from the GP solution decrease dramatically.

Author: AlexandreSinger

vpr/src/base/flat_placement_types.h

@@ -35,6 +35,16 @@ struct t_flat_pl_loc {
         return *this;
     }
 
+    /**
+     * @brief Subtracts the coordinates of another t_flat_pl_loc to this one.
+     */
+    t_flat_pl_loc& operator-=(const t_flat_pl_loc& other) {
+        x -= other.x;
+        y -= other.y;
+        layer -= other.layer;
+        return *this;
+    }
+
     /**
      * @brief Divides the coordinates of this t_flat_pl_loc by a divisor.
      *