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.