Decide how to deal with environment issues during parameter extraction

The parameter extraction procedure requires that xcetool runs the notebook up to the parameters cell. This could potentially require elements of the notebook's expected environment (Python packages and environment variables) to be present in the environment where xcetool is running. In the most extreme case, the entire notebook environment might be required. In many cases that's not a problem -- e.g. in a typical Jupyter Lab environment xcetool may run in a Jupyter Lab terminal tab in exactly the same environment as the notebook. But there are other scenarios. Initial thoughts:

1. This requirement, and potential solutions to related problems, should be clearly documented. Generally some simple notebook rearrangement (parameter cell as early as possible) should be enough.
2. It might be worth providing something like a --continue option which would ignore errors during parameter extraction and make a best effort, and/or an option to ignore the pre-parameter-cell code entirely. In many (most?) cases this should be sufficient.

Note that this topic concerns the environment in which xcetool runs, not the environment in which the exported script will run.