remove port specification

[vsoch]

I don't see any logic for asking the user to pre-generate a custom port, and then giving an error message if/when another notebook (by the same user) is opened. It would make more sense to generate a port on the fly, and risk that another user is using it.

I'll add this tweak to my current PR.

[vsoch]

oh hmm, now I see that it's required to resume and end! Need to think about this. Do you have any ideas for how this could be done?

[vsoch]

I was thinking that the user could just specify it as an argument, but that makes usage too hard I think. For the record, if we need to generate a random port on the fly:

$ shuf -i 49152-65335 -n 1