Use system site packages from virtualenv to avoid needing numpy reinstalls

Aaron and Uma;
It sounds like the major installation woe is needing an isolated numpy installation inside of the gemini virtualenv. This fix allows use of global site packages in addition to the virtualenv, so we can skip numpy if already present. The downside is we are not totally isolated so can still have some version conflicts on tricky systems but overcoming the installation difficulties seems like the number one goal. The requirements file no longer specifies exactly needed versions to also play better with previously installed libraries.