Add tensor scan op

This is the initial implementation a generic scan operation.
Based on this, cumsum and cumprod method have been added to TensorBase.

unsupported/Eigen/CXX11/src/Tensor/README.md

@@ -1168,6 +1168,44 @@ Reduce a tensor using a user-defined reduction operator.  See ```SumReducer```
 in TensorFunctors.h for information on how to implement a reduction operator.
 
 
+## Scan Operations
+
+A *Scan* operation returns a tensor with the same dimensions as the original
+tensor. The operation performs an inclusive scan along the specified
+axis, which means it computes a running total along the axis for a given
+reduction operation.
+If the reduction operation corresponds to summation, then this computes the
+prefix sum of the tensor along the given axis.
+
+Example:
+dd a comment to this line
+
+    // Create a tensor of 2 dimensions
+    Eigen::Tensor<int, 2> a(2, 3);
+    a.setValues({{1, 2, 3}, {4, 5, 6}});
+    // Scan it along the second dimension (1) using summation
+    Eigen::Tensor<int, 2> b = a.cumsum(1);
+    // The result is a tensor with the same size as the input
+    cout << "a" << endl << a << endl << endl;
+    cout << "b" << endl << b << endl << endl;
+    =>
+    a
+    1 2 3
+    6 5 4
+
+    b
+    1  3  6
+    4  9 15
+
+### <Operation> cumsum(const Index& axis)
+
+Perform a scan by summing consecutive entries.
+
+### <Operation> cumprod(const Index& axis)
+
+Perform a scan by multiplying consecutive entries.
+
+
 ## Convolutions
 
 ### <Operation> convolve(const Kernel& kernel, const Dimensions& dims)