Node names and other stuff

[ollmer]

• Auto node names from classes if not set explicitly
• Subagent node with the name derived from subagent name
• Named inputs for subagents
• Readme example from the intro
• prompt_with_guidance() convenience wrapper function
• hello_world.py example with the same example as item 1 in intro and readme.
• move usermodel classes from the core to the example
• test for make traces example do not override the test resources anymore
• simplified API keys handling in the intro notebook

[rizar]

LGTM! I'm a little bit apprehensive about having links from the nodes to the subagents. This can have nasty effects for agent serialization. But Chain is not a very high priority at the moment, so can address this later. Also we are not yet sure who for what will be using agent serialization....