PyPI install needs to come with disclaimer to get mayavi separately

[kccwing]

current PR #320 is configured to give two versions of wheels for pip install, github version includes pypi dependency, and PyPI version has it removed. updates in documentation (and perhaps at install?) needed

https://stackoverflow.com/questions/68073819/pypi-install-requires-direct-links

https://github.com/jcrozum/pystablemotifs/blob/7547190f3f034a4aa8895995662645eed4fea9e2/README.md