Add a GPIO Group to STM32 GPIO

[strongly-typed]

How and where should a test to the CI be added?

[salkinium]

See http://modm.io/guide/testing/.
You can just add it in the generic folder and it'll just get compiled, just like before. Manual call:

 cd examples
 ../tools/scripts/examples_compile.sh generic

I plan to compile the generic examples for all boards at least, but that's not been implemented yet.

[salkinium]

I don't agree with this PR on the point that the canonical interface of this is modm::platform::SoftwareGpioPort. I would rather see that improved to merge port access in a generic way than introduce such a specific new interface and implementation.
I'd recommend you keep this in your own code base/fork for now while I check how difficult it is to implement this generically in SoftwareGpioPort.

[salkinium]

while I check how difficult it is to implement this generically in SoftwareGpioPort.

This is fairly easy to implement with C++17 fold expressions, which in combination with the relaxed constexpr function rules, flattens the recursive requirement of variadic arguments. constexpr if makes the resulting code gen very efficient. Is using C++17 an issue for your use-case?

[salkinium]

I'm closing this in favor of #19.