sparse module: add support for umfpack, the sparse direct LU

solver from suitesparse (as cholmod). It seems to be even faster
than SuperLU and it was much simpler to interface ! Well,
the factorization is faster, but for the solve part, SuperLU is
quite faster. On the other hand the solve part represents only a
fraction of the whole procedure. Moreover, I bench random matrices
that does not represents real cases, and I'm not sure at all
I use both libraries with their best settings !

Eigen/Sparse

@@ -50,6 +50,10 @@
   namespace Eigen { struct SluMatrix; }
 #endif
 
+#ifdef EIGEN_UMFPACK_SUPPORT
+  #include "umfpack.h"
+#endif
+
 namespace Eigen {
 
 #include "src/Sparse/SparseUtil.h"
@@ -79,6 +83,10 @@ namespace Eigen {
 # include "src/Sparse/SuperLUSupport.h"
 #endif
 
+#ifdef EIGEN_UMFPACK_SUPPORT
+# include "src/Sparse/UmfPackSupport.h"
+#endif
+
 } // namespace Eigen
 
 #endif // EIGEN_SPARSE_MODULE_H