-new: recursive costs system, useful to determine automatically

when to evaluate arguments and when to meta-unroll.
-use it in Product to determine when to eval args. not yet used
 to determine when to unroll. for now, not used anywhere else but
 that'll follow.
-fix badness of my last commit

doc/examples/class_CwiseBinaryOp.cpp

@@ -1,26 +1,29 @@
+// FIXME - this example is not too good as that functionality is provided in the Eigen API
+// additionally it's quite heavy. the CwiseUnaryOp example is better.
+
 #include <Eigen/Core>
 USING_PART_OF_NAMESPACE_EIGEN
 using namespace std;
 
 // define a custom template binary functor
-struct CwiseMinOp EIGEN_EMPTY_STRUCT {
-    template<typename Scalar>
+template<typename Scalar> struct CwiseMinOp EIGEN_EMPTY_STRUCT {
     Scalar operator()(const Scalar& a, const Scalar& b) const { return std::min(a,b); }
+    enum { Cost = Eigen::ConditionalJumpCost + Eigen::NumTraits<Scalar>::AddCost };
 };
 
 // define a custom binary operator between two matrices
 template<typename Derived1, typename Derived2>
-const Eigen::CwiseBinaryOp<CwiseMinOp, Derived1, Derived2>
+const Eigen::CwiseBinaryOp<CwiseMinOp<typename Derived1::Scalar>, Derived1, Derived2>
 cwiseMin(const MatrixBase<Derived1> &mat1, const MatrixBase<Derived2> &mat2)
 {
-  return Eigen::CwiseBinaryOp<CwiseMinOp, Derived1, Derived2>(mat1, mat2);
+  return Eigen::CwiseBinaryOp<CwiseMinOp<typename Derived1::Scalar>, Derived1, Derived2>(mat1, mat2);
 }
 
 int main(int, char**)
 {
   Matrix4d m1 = Matrix4d::random(), m2 = Matrix4d::random();
   cout << cwiseMin(m1,m2) << endl;            // use our new global operator
-  cout << m1.cwise<CwiseMinOp>(m2) << endl;   // directly use the generic expression member
-  cout << m1.cwise(m2, CwiseMinOp()) << endl; // directly use the generic expression member (variant)
+  cout << m1.cwise<CwiseMinOp<double> >(m2) << endl;   // directly use the generic expression member
+  cout << m1.cwise(m2, CwiseMinOp<double>()) << endl; // directly use the generic expression member (variant)
   return 0;
 }