Fix JoinAnswer/JoinNode

[MichelBartels]

Proposed changes:
As described in #2599 and #2593, there are currently issues with nodes that can accept multiple inputs. This PR addresses these by changing the nodes and not the pipelines like in #2593.

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code
• Enable actions on your fork
• Added tests
• Updated documentation

[ZanSara]

@MichelBartels I had a look at the changes, good progess! I will review them shortly, but first of all let me highlight one thing: let's not mess with the BaseComponent.run() signature. It's unfixable in its current form, even adding **kwargs is going to make mypy go mad. Please leave it alone 🙏

JoinNode.run() method should simply silence the mypy error. It's really ugly, but that's how BaseComponent is right now. Sorry for that 😅 On the same note, the changes in BaseComponent about take_kwargs are also better kept for a more comprehensive PR about improving the situation with BaseComponent, unless you think those kwargs are really necessary to some node. In that case, let's review why: **kwargs are dangerous in the current situation.

I will link this PR to the ones related to BaseComponent.run() as yet another proof that stuff here needs a good rewrite fast...

[ZanSara]

Hey @MichelBartels how about this one? Shall I take care of it? It's not urgent but I'd like to see this in master 🙂

[MichelBartels]

Hi @ZanSara, sorry forgot a bit about this. I have now done your suggested changes. kwargs is now no longer used. Not using it will probably make things more cumbersome in the future, but I understand that it makes sense to wait.

[ZanSara]

Nice! Thank you very much for this PR 😊 Hopefully the issues you had to work around will be fixed very soon.