Replace --python option with a --env option

[amol-]

See #655 for prior discussion.

As --python doesn't influence anymore the python interpreter version used to run the content, the only reason why the option is still around is to inspect a specific python environment to replicate the libraries installed there (generate requirements.txt based on that environment).

The naming of the option at that point is confusing, and renaming it to --env which points to the python environment to inspect would better comunicate the intent of the option.