Add run workflow function

[znicholls]

Add a run workflow function to create a simpler Python API (useful for writing example notebooks)

• [x] Tests added (N/A)
• [x] Documentation added
• [x] Example added (in the documentation, to an existing notebook, or in a new notebook) (N/A)
• [x] Description in CHANGELOG.rst added (single line such as: (`#XX <https://github.com/iiasa/climate-assessment/pull/XX>`_) Added feature which does something)

Depends on:

• [x] #4

[znicholls]

@phackstock @jkikstra first one here