Add lp_solver_wrapper.h to work doxygen.

Add empty docs in order to make the functions and classes visible in
doxygen.

An experimental LPSolverWrapper class which uses glpk. For a short
demo, max flow problems are solved with it. This demo does not
demonstrates, but the main aims of this class are row and column
generation capabilities, i.e. to be a core for easily
implementable branch-and-cut a column generetion algorithms.