Check virtual environment is active

[hombit]

Change Description

This PR should prevent user from installing stuff into their global Python paths. It does it in two ways:

1. Adds a note about this in both Readme and copier output
2. Checks if conda or venv is active in .prepare_project.sh. It also changes pip to python -m pip to be sure that pip from virtual environment is used, not the global one.

Fixes #399

Checklist

• This PR is meant for the lincc-frameworks/python-project-template repo and not a downstream one instead.
• This change is linked to an open issue
• This change includes integration testing, or is small enough to be covered by existing tests

[drewoldag]

Thanks for already addressing the naming suggestions. I've left one final comment/concern about the conditional check in .prepare_project.sh. Once I get your feedback on that, then I'm happy to give this the ✔️

[drewoldag]

Cool, this looks good to me. Thanks for making the changes!