Fix plyvel installs on MacOS

[englehardt]

MacOS users seem to run into problems when trying to install plyvel from pip, and instead must install it from brew. See this install script as an example. We should figure out how to address this in the PyPI package.

[birdsarah]

Can we remove the dependency on plyvel in openwpm? It is part of the cross platform challenge of running openwpm on windows.