SHARED_PORT currently limited to UserCLK

[EverythingElseWasAlreadyTaken]

The SHARED_PORT is currently limited to UserCLK.
Every port that is marked with SHARED_PORT, will be automaitcally connected to the UserCLK:
https://github.com/FPGA-Research/FABulous/blob/main/FABulous/fabric_generator/gen_fabric/gen_fabric.py#L356

I'm not sure if it even checks if its an input or output.

This behaviour should be checked and documented, the current documentation about it is misleading.
https://fabulous.readthedocs.io/en/latest/user_guide/building_doc/fabric_definition.html#primitives

Also, the general concept of the SHARED_PORTS should be evaluated again.