Fix yaml issues with BBH ID example nb

[vaishakp]

Proposed changes

1. Fix loading and writing of yaml files with multiple yaml documents in the same stream. Previously, the existing tutorial presumably worked with a single document yaml stream. The latest BinaryBlackHole.yaml has multiple docs. This change allows such streams to be read and edited in this example notebook.

2. Make minor changes in path names to make it more informative.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[nilsvu]

Hey Vaishak! Thanks for looking at this. Indeed this tutorial is very out of date. I was going to replace it by the one from SXScon.

[vaishakp]

Hey Vaishak! Thanks for looking at this. Indeed this tutorial is very out of date. I was going to replace it by the one from SXScon.

The one from SXSCon is nice and easy for BBH ID+evolution at the highest level. I however find that this one is informative and could be preserved as an ID example that works at a slightly lower level, working directly with the input files. In my opinion, it will be useful to keep both.