Restrict input data to have at most one connection

[liamhuber]

Closes #498. This doesn't make the graph truly deterministic yet, but it does resolve the explicit non-determinacy mentioned there.

The goes part of the way to resolving #502, but there we need what is currently the sole input "connection" and the current concept of a "value_receiver" (well, "value_source" from the perspective of the input channel) to be put on more equal footing -- i.e. each data node can have at most one upstream "source". So this is related PR, but I leave that issue open.

Even though making multiple connections is now strictly illegal, when we go via the route of assignment to input panels there is syntactic sugar to do the necessary disconnection for you. IMO this should feel very pythonic, as when we assign something new to the attribute of course the old assignment is thrown away. E.g.

import pyiron_workflow as pwf

wf = pwf.Workflow("single_input")
wf.n1a = pwf.std.UserInput(0)
wf.n1b = pwf.std.UserInput(100)
wf.n2 = pwf.std.UserInput()

wf.n2.inputs.user_input = wf.n1a
wf.n2.inputs.user_input = wf.n1b
wf()
>>> {'n1a__user_input': 0, 'n2__user_input': 100}

Where we see n2 is correctly leveraging its one and only more-recent source, and we can see that the old source got disconnected because the workflow can tell it's a dangling output.

This means the "manually constructed while-loop" demo had to be thrown out of the integration tests, because the whole concept of relying on the "input connection stack" no longer exists. This should be fine, as while-loop behaviour has its own class now (#650)

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[github-actions]

👈 Launch a binder notebook on branch pyiron/pyiron_workflow/single_input

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.85%. Comparing base (90c9fa1) to head (f4583bf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #673      +/-   ##
==========================================
- Coverage   92.05%   91.85%   -0.20%     
==========================================
  Files          34       34              
  Lines        3576     3587      +11     
==========================================
+ Hits         3292     3295       +3     
- Misses        284      292       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.