Add a convenience function to get a GraphBuilder instance directly from a lsl.Model

[jobrachem]

Currently, if you want to get a GraphBuilder from a model, the process looks like this:

1. Extract the nodes and vars from the model via lsl.Model.copy_nodes_and_vars()
2. Initializie a GraphBuilder
3. Add the nodes and vars to the GraphBuilder via .add() In this step, you need some Python experience about how to get the output of lsl.Model.copy_nodes_and_vars() into the necessary format accepted by .add()

While this is not impossible, I feel like it would be much more user-friendly to provide a method that enables users to directly obtain a fully filled GraphBuilder instance directly from a model.