COIN-OR Clp support

[Sebastian Raaphorst]

Clp, in some experiments, was shown to outperform all major free LP solvers. We should create a plugin to use Clp:

https://projects.coin-or.org/Clp

This would entail making a subclass of LPSolver (see LPSolver.h and LPSolver.cpp, or the implementations in src for GLPKSolver and CPLEXSolver.)