Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/unsupported/doc/examples/BVH_Example.cpp

@@ -0,0 +1,71 @@
+#include <Eigen/StdVector>
+#include <iostream>
+#include <unsupported/Eigen/BVH>
+
+using namespace Eigen;
+typedef AlignedBox<double, 2> Box2d;
+
+namespace Eigen {
+Box2d bounding_box(const Vector2d &v) {
+  return Box2d(v, v);
+}  // compute the bounding box of a single point
+}
+
+struct PointPointMinimizer  // how to compute squared distances between points
+                            // and rectangles
+{
+  PointPointMinimizer() : calls(0) {}
+  typedef double Scalar;
+
+  double minimumOnVolumeVolume(const Box2d &r1, const Box2d &r2) {
+    ++calls;
+    return r1.squaredExteriorDistance(r2);
+  }
+  double minimumOnVolumeObject(const Box2d &r, const Vector2d &v) {
+    ++calls;
+    return r.squaredExteriorDistance(v);
+  }
+  double minimumOnObjectVolume(const Vector2d &v, const Box2d &r) {
+    ++calls;
+    return r.squaredExteriorDistance(v);
+  }
+  double minimumOnObjectObject(const Vector2d &v1, const Vector2d &v2) {
+    ++calls;
+    return (v1 - v2).squaredNorm();
+  }
+
+  int calls;
+};
+
+int main() {
+  typedef std::vector<Vector2d, aligned_allocator<Vector2d> >
+      StdVectorOfVector2d;
+  StdVectorOfVector2d redPoints, bluePoints;
+  for (int i = 0; i < 100;
+       ++i) {  // initialize random set of red points and blue points
+    redPoints.push_back(Vector2d::Random());
+    bluePoints.push_back(Vector2d::Random());
+  }
+
+  PointPointMinimizer minimizer;
+  double minDistSq = std::numeric_limits<double>::max();
+
+  // brute force to find closest red-blue pair
+  for (int i = 0; i < (int)redPoints.size(); ++i)
+    for (int j = 0; j < (int)bluePoints.size(); ++j)
+      minDistSq = std::min(minDistSq, minimizer.minimumOnObjectObject(
+                                          redPoints[i], bluePoints[j]));
+  std::cout << "Brute force distance = " << sqrt(minDistSq)
+            << ", calls = " << minimizer.calls << std::endl;
+
+  // using BVH to find closest red-blue pair
+  minimizer.calls = 0;
+  KdBVH<double, 2, Vector2d> redTree(redPoints.begin(), redPoints.end()),
+      blueTree(bluePoints.begin(), bluePoints.end());  // construct the trees
+  minDistSq =
+      BVMinimize(redTree, blueTree, minimizer);  // actual BVH minimization call
+  std::cout << "BVH distance         = " << sqrt(minDistSq)
+            << ", calls = " << minimizer.calls << std::endl;
+
+  return 0;
+}