Provide a `requirements.txt` file for installing with pip

[ianmkenney]

I don't think we have anything that would break by using pip only. I would still encourage using environments.

[ianmkenney]

from the current conda env file, the corresponding requirements.txt should be

MDAnalysis
MDAnalysisTests
MDAnalysisData
nglview
notebook<7
jupyter_contrib_nbextensions
importlib-metadata
rise
pytest
nbval
ipywidgets<8