Several improvements in sparse module:

* add a LDL^T factorization with solver using code from T. Davis's LDL
  library (LPGL2.1+)
* various bug fixes in trianfular solver, matrix product, etc.
* improve cmake files for the supported libraries
* split the sparse unit test
* etc.

Eigen/src/Sparse/CholmodSupport.h

@@ -155,6 +155,7 @@ void SparseLLT<MatrixType,Cholmod>::compute(const MatrixType& a)
   }
 
   cholmod_sparse A = const_cast<MatrixType&>(a).asCholmodMatrix();
+  // TODO
   if (m_flags&IncompleteFactorization)
   {
     m_cholmod.nmethods = 1;