Make bdsg Python module installable via Pip on Mac

[adamnovak]

This makes the bdsg module that pip install --upgrade bdsg will produce actually work.

At the end of the CMake build, setup,py now just goes in and rewrites all the references between libraries to use @loader_path and thus be relative to the file asking for the load. That's how Numpy's pre-built Mac wheels get their libraries to find each other.

I'm also (in the Mac wheels only, for now), eliminating all the versioned symlinks and just pointing everything to the actual files, because in a wheel those symlinks become duplicate copies of the libraries, because (I think) zip files can't do symlinks. This will make the installed size smaller.

I also make an attempt to build with the right number of threads for your machine now, using code I wrote for Toil.

This fixes #83.