* Check for a valid python installation
* Actually exit if `venv` directory already exists
* Don't require virtualenv package (use `venv` builtin)
* Ensure that we don't attempt to recompile numpy or pandas when
installing on more recent versions of Mac
This simplifies the multiple choice installation process into
a simplified version which uses a local virtualenv, which will
hopefully be easier to support and debug.
Daniel Thorn
Will Lachance