added an experimental IterativeSolvers module (currently in unsupported)

with a constrained conjugate gradient algorithm adapted from GMM++/ITL.
This algorithm is needed for Step.

doc/unsupported_modules.dox

@@ -13,4 +13,7 @@ namespace Eigen {
 /** \ingroup Unsupported_modules
   * \defgroup AdolcForward_Module Adolc forward module */
 
+/** \ingroup Unsupported_modules
+  * \defgroup IterativeSolvers_Module Iterative solvers module */
+
 } // namespace Eigen