add experimental code for sparse matrix:

- uses the common "Compressed Column Storage" scheme
 - supports every unary and binary operators with xpr template
   assuming binaryOp(0,0) == 0 and unaryOp(0) = 0 (otherwise a sparse
   matrix doesnot make sense)
 - this is the first commit, so of course, there are still several shorcommings !

Eigen/Sparse

@@ -0,0 +1,16 @@
+#ifndef EIGEN_SPARSE_MODULE_H
+#define EIGEN_SPARSE_MODULE_H
+
+#include "Core"
+#include <vector>
+#include <map>
+
+namespace Eigen {
+
+#include "src/Sparse/SparseArray.h"
+#include "src/Sparse/SparseMatrix.h"
+#include "src/Sparse/CoreIterators.h"
+
+} // namespace Eigen
+
+#endif // EIGEN_SPARSE_MODULE_H