ADD: new track message, Entity class and Position class

libs/eigen/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:
+ - \c Index \c rows() and \c Index \c cols(): returns number of rows and columns respectively
+ - \c operator* with your type and an %Eigen dense column vector (its actual implementation goes in a specialization of the internal::generic_product_impl class)
+
+\c Eigen::internal::traits<> must also be specialized for the wrapper type.
+
+Here is a complete example wrapping an Eigen::SparseMatrix:
+\include matrixfree_cg.cpp
+Output: \verbinclude matrixfree_cg.out
+
+*/
+
+}