Add matrix-free solver example

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diff --git a/doc/MatrixfreeSolverExample.dox b/doc/MatrixfreeSolverExample.dox
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+
+namespace Eigen {
+
+/**
+
+\eigenManualPage MatrixfreeSolverExample Matrix-free solvers
+
+Iterative solvers such as ConjugateGradient and BiCGSTAB can be used in a matrix free context. To this end, user must provide a wrapper class inheriting EigenBase<> and implementing the following methods:
+ - Index rows() and Index cols(): returns number of rows and columns respectively
+ - operator* with and %Eigen dense column vector (its actual implementation goes in a specialization of the internal::generic_product_impl class)
+
+Eigen::internal::traits<> must also be specialized for the wrapper type.
+
+Here is a complete example wrapping a Eigen::SparseMatrix:
+\include matrixfree_cg.cpp
+Output: \verbinclude matrixfree_cg.out
+
+*/
+
+}
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