Fix bug #635: add isCompressed to MappedSparseMatrix for compatibility

(transplanted from b72a686830
)

Eigen/src/SparseCore/MappedSparseMatrix.h

@@ -51,6 +51,8 @@ class MappedSparseMatrix
     inline Index innerSize() const { return m_innerSize; }
     inline Index outerSize() const { return m_outerSize; }
     
+    bool isCompressed() const { return true; }
+
     //----------------------------------------
     // direct access interface
     inline const Scalar* valuePtr() const { return m_values; }