CHOLMOD: provide access to the cholmod_common object

This allows other implementations which are using the istl-cholmod wrapper
to set options and other paramters of the cholmod solver

CHANGELOG.md

 # Master (will become release 2.8)
 
+- Added public access of the `cholmod_common` object in class `Cholmod`.
+
 ## Deprecations and removals
 - Drop deprecated bindings of direct solver Pardiso.
 

dune/istl/cholmod.hh

@@ -311,6 +311,16 @@ public:
     return SolverCategory::Category::sequential;
   }
 
+  /** \brief return a reference to the CHOLMOD common object for advanced option settings
+   *
+   *  The CHOLMOD common object stores all parameters and options for the solver to run
+   *  and can be modified in several ways, see CHOLMOD Userguide for further information
+   */
+  cholmod_common& cholmodCommonObject()
+  {
+      return c_;
+  }
+
 private:
 
   // create a destrucable unique_ptr