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/src/Sparse/SuperLUSupport.h

@@ -311,8 +311,8 @@ void SparseLU<MatrixType,SuperLU>::compute(const MatrixType& a)
           &m_sluStat, &info, Scalar());
   StatFree(&m_sluStat);
 
-  // FIXME how to check for errors ???
-  Base::m_succeeded = true;
+  // FIXME how to better check for errors ???
+  Base::m_succeeded = (info == 0);
 }
 
 // template<typename MatrixType>
@@ -362,6 +362,8 @@ bool SparseLU<MatrixType,SuperLU>::solve(const MatrixBase<BDerived> &b, MatrixBa
     &m_sluFerr[0], &m_sluBerr[0],
     &m_sluStat, &info, Scalar());
   StatFree(&m_sluStat);
+
+  return info==0;
 }
 
 #endif // EIGEN_SUPERLUSUPPORT_H