Add API for IQ channel

Created by: mjhaye

This is how I have done it now for lls. Still based on the IQ_channel_constructor to keep the implementation identical, but maybe a new/double implementation is preferred (to avoid circular dependencies).

Also the name define_iq_channel looks a lot like define_IQ_channel but I could not think of something better and eventually the latter should be removed, I think.

Also added py.typed so that the typehints that exist at least are available to external users.