Added Restaint.h containing struct; this should have been added earlier

src/Restraint.h

+// Exclude.h
+// Copyright Justin Dufresne and Terrance Howard, 2013
+
+#pragma once
+#include "useful.h"
+
+struct Restraint {
+public:
+    Restraint() : id(-1) {}
+    Restraint(int id, Vector3 r0, int k) : id(id), r0(r0), k(k) {}
+    int id;
+    Vector3 r0;
+    float k;
+};