Add check if all blueprint ports are connected

[kairichard]

The Problem

Unconnected outports of blueprints lead to two undesired effects:

• if we pull on the port it will block all other data flowing forward on that port
• exposing ports to the user that actually never carry data

Solution

Before the blueprint is started check if all outports are connected. Easier said than done. For the edge case, that operator running is for actually that one -> https://bitspark.de/slang/docs/operator/iterate with a connected delegate, the ports appear to be not connected because of src being empty. But it still works.

Todo

• [ ] figure out why delegates ports have no src

@td5r Ideas on how this can be done in a more robust way?

• fixes #235