Import Experiment from README directly

At the moment, you need to load the actual data to be able to extract a PyBaMM Experiment object, but you can't do it from the REDME.

The goal is to add this functionality to the `readme_processor` module.

Start by inspecting the readme_processor module and its current path for extracting a PyBaMM Experiment after data is loaded. Trace how the README is parsed and identify the existing Experiment extraction entry point. Done means the module can produce the PyBaMM Experiment directly from the README without loading the actual data.

Written by the indexing model from the issue text.