Add RK6 with residual viscosity update and reduce computational effort of spatial scheme - Add RK6 with residual updates - Change the D2 operator for upwind schemes to one less computationally expensive.

% Converts a gridfunction to a matrix
% Takes a grid function and and a structured grid.
function F = funcToMatrix(g, gf)
    F = reshapeRowMaj(gf, g.size());
end