Performance test comparison with muParser.

Simple performance comparison between EigenLab and muParser. To
summarize, if you can evaluate your expression on one big matrix of
values, then EigenLab is incredibly fast as it takes advantage of
Eigen’s efficient matrix manipulations. However, if you need to
evaluate your expression many separate times on a scalar value, then
muParser is your best choice in terms of speed. This script is based on
one by Ilja Honkonen <ilja.j.honkonen@nasa.gov>.