Use KLU mex interface in mplinsolve for GNU Octave support

[rwl]

The lu function in GNU Octave uses UMFPACK as a solver. As previously discussed on the mailing lists, there are performance benefits to be found using KLU:

https://www.mail-archive.com/matpower-dev-l@cornell.edu/msg00006.html
https://www.mail-archive.com/matpower-l@cornell.edu/msg03040.html

GNU Octave doesn't follow the same rules as MATLAB for switching to the Gilbert-Peierls algorithm when lu is called with a certain number of arguments. However, KLU includes a mex interface that could be called explicitly if installed. This would make the improvements that were introduced in MATPOWER/matpower#7 available when running Octave.