Identify the best solver currently installed on the system and specify that
it should be used to solve a conservation planning problem().
Ranked from best to worst, the available solvers that can be used are:
add_gurobi_solver(), add_cplex_solver(), add_rsymphony_solver(),
and finally add_lpsymphony_solver().
add_default_solver(x, ...)problem() (i.e. '>ConservationProblem) object.
arguments passed to the solver.
Object (i.e. '>ConservationProblem) with the solver
added to it.