Managing conda/pip hybrid python environment

Move the post-PR discussion of conda/pip hybrid environment https://github.com/aiidalab/aiidalab-docker-stack/pull/338#issuecomment-1331199302 as an issue here for further discussion.

Summary:
- Not a very good idea to [mix use pip in a conda environment](https://www.anaconda.com/blog/using-pip-in-a-conda-environment)
- Have to do it at the moment in AiiDAlab since we want to make it is possible that app can be install without publishing the PyPI or conda-forge. This reduces the burden for app developers.
- The principle is conda only for packages that need to be compiled (how we know this in advance, for instance, the `pymatgen`) and pip for or regular python packages.