Improve efficiency of path-following for QRE tracing.

Improves the efficiency of the Python implementation of path-following,
used in computing QRE.  Primarily, this is done by taking greater
advantage of numpy to handle linear algebra rather than looping.
Speedups over the previous implementation are substantial.  Testing
has been done (primarily on some all-pay auction examples) to confirm
that the old and new implementations return the same results.