Time-independent Usage for MOLFiniteDifference()

The docs should be up soonish. For the answer right now though, see https://github.com/SciML/MethodOfLines.jl/blob/master/test/pde_systems/MOL_NonlinearProblem.jl#L64-L65 . The result of a time-independent problem is a NonlinearProblem to be solved via NonlinearSolve.jl.