Enhancements to MatrixProduct

This patch is a bit huge so I will try to summarize and comment:

• 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.

Comments:

Most tests that were failing are now ok, except mixingtypes which needs to be implemented. The ones that are failing are doing so because there's no storePacketBlock for Incr != 1, the implementation for other increments is pretty slow so I'll leave up to the community to decide if we end up doing it or not.

I'm aware that I used a union against community standards but unfortunately the appropriate approach produces a strict-alignment warning and it's pretty cumbersome, I don't see a way to fix this for now.