* Draft of a eigenvalues solver

(does not support complex and does not re-use the QR decomposition)

* Rewrite the cache friendly product to have only one instance per scalar type !
  This significantly speeds up compilation time and reduces executable size.
  The current drawback is that some trivial expressions might be
  evaluated like conjugate or negate.

* Renamed "cache optimal" to "cache friendly"

* Added the ability to directly access matrix data of some expressions via:
  - the stride()/_stride() methods
  - DirectAccessBit flag (replace ReferencableBit)

Eigen/src/Core/Triangular.h

@@ -67,7 +67,7 @@ struct ei_traits<Triangular<Mode, MatrixType> >
     ColsAtCompileTime = MatrixType::ColsAtCompileTime,
     MaxRowsAtCompileTime = MatrixType::MaxRowsAtCompileTime,
     MaxColsAtCompileTime = MatrixType::MaxColsAtCompileTime,
-    Flags = (_MatrixTypeNested::Flags & ~(VectorizableBit | Like1DArrayBit)) | Mode,
+    Flags = (_MatrixTypeNested::Flags & ~(VectorizableBit | Like1DArrayBit | DirectAccessBit)) | Mode,
     CoeffReadCost = _MatrixTypeNested::CoeffReadCost
   };
 };