Cyphal/UDP transfer <-> address/port conversion

[asmfreak]

For #98 the specification in pycyphal does not specify conversion between transfer to address and port.

It would be great to integrate this documentation with this part in the first text part of the specification.

Maybe something like this:

Transfer kind	Address	Port
Message	Multicast base + subnet id + subject id	SUBJECT_PORT = 16383
Service	Target address	SERVICE_BASE_PORT = 16384; TARGET_PORT = SERVICE_BASE_PORT + service_id * 2 + responce_not_request

[pavel-kirienko]

Superseded by #98