Add a proof concept API to configure the blocking parameters at runtime.

After validation of the final API I'll update the other products to use it.

Eigen/src/Core/util/BlasUtil.h

@@ -139,7 +139,7 @@ struct ei_product_blocking_traits
     mr = 2 * PacketSize,
 
     // max cache block size along the K direction
-    Max_kc = 8 * ei_meta_sqrt<EIGEN_TUNE_FOR_CPU_CACHE_SIZE/(64*sizeof(Scalar))>::ret,
+    Max_kc = 4 * ei_meta_sqrt<EIGEN_TUNE_FOR_CPU_CACHE_SIZE/(64*sizeof(Scalar))>::ret,
 
     // max cache block size along the M direction
     Max_mc = 2*Max_kc

Eigen/src/Core/util/Constants.h

@@ -269,6 +269,8 @@ namespace Architecture
 
 enum { CoeffBasedProductMode, LazyCoeffBasedProductMode, OuterProduct, InnerProduct, GemvProduct, GemmProduct };
 
+enum Action {GetAction, SetAction};
+
 /** The type used to identify a dense storage. */
 struct Dense {};