Proposed interface allowing user choice of advection scheme

[xtalax]

    MOLFiniteDifference(dxs, time=nothing;
                        approx_order = 2, advection_scheme = UpwindScheme(),
                        grid_align = CenterAlignedGrid(), kwargs...)

advection_scheme: The scheme to be used to discretize advection terms, i.e. first order spatial derivatives and associated coefficients. Defaults to UpwindScheme(). This is the only relevant scheme at present. A coming PR will add WENOScheme().

Is this a good interface? Should this default to WENO in future?
@ChrisRackauckas

[ChrisRackauckas]

Yeah I think a WENO default could be good.

[xtalax]

closed by #132