Add support and/or instructions for cases where user model and/or wrapper needs to use a different python environment than boa

[jemissik]

A user's model may need to run in a different python environment than boa's environment, since there could be conflicts that make it impossible to install boa and the model in the same environment.

A user's wrapper could also have this problem (e.g. if it needs to import from a model that needs a different environment). In this case it could simply be treated the same way as other external wrappers, but perhaps there would still be conveniences over non-python wrappers. As one idea it would probably be nicer and more functional to use pickles instead of needing to convert everything to json.

We should decide the best way to handle these cases and add instructions to the docs.