First step in fixing issue #3157 is to check-in the source code and hook
it into our build system.
The inclusion of the MuParser source adds the MIT License to those that
apply to fish. Update our documentation to reflect that fact.
The MuParser documentation is at
http://beltoforion.de/article.php?a=muparser. The source was downloaded
from https://github.com/beltoforion/muparser/releases. It is also hosted
on Github, https://github.com/beltoforion/muparser/. I did not download
it from Github because that source contained just a couple of cleanup
changes which don't affect its behavior.
