external flag for broker makes non-ZeroMQ cores not work

[nightlark]

Describe the bug
Starting the broker with --external with a core type other than ZeroMQ causes the broker to not work (federates are unable to connect).

What is the expected behavior?
The other core types should continue working when given --external (they don't seem to need it, so it should either be ignored or it should be removed from the help message of non-ZMQ cores and give an error/warning when used).

Environment (please complete the following information):

• HELICS version: 2.0.0 42cff6f

[nightlark]

This also applies when using --ipv4 (and I imagine --ipv6 as well).

[phlptp]

Have you tried this with the UDP broker?

[nightlark]

I don't remember. The TCP core definitely wasn't working.

[phlptp]

I was able to verify the TCP issue as well, but the UDP core works differently so I wasn't sure if that fails in this context as well. I will test it soon, but I wondered if you knew.

[phlptp]

Ok, @nightlark can you check again.

[nightlark]

I tested the tcp, tcpss, and udp cores; the --external flag no longer breaks any of them.

[phlptp]

closed with PR #720