MatrixProuct enhancements:

- Changes to Altivec/MatrixProduct
  Adapting code to gcc 10.
  Generic code style and performance enhancements.
  Adding PanelMode support.
  Adding stride/offset support.
  Enabling float64, std::complex and std::complex.
  Fixing lack of symm_pack.
  Enabling mixedtypes.
- Adding std::complex tests to blasutil.
- Adding an implementation of storePacketBlock when Incr!= 1.

test/blasutil.cpp

@@ -200,5 +200,7 @@ EIGEN_DECLARE_TEST(blasutil)
 
         CALL_SUBTEST_5(run_test<float_t>());
         CALL_SUBTEST_6(run_test<double_t>());
+        CALL_SUBTEST_7(run_test<std::complex<float> >());
+        CALL_SUBTEST_8(run_test<std::complex<double> >());
     }
 }