I have a problem of using `DiffeqOperators.jl`

on non-uniform grid. Here is an example of a simple diffusion equation D\nabla^2C+1=0 between [0,50] with Robin boundary conditions on both side. It is solved using centered spatial difference and Method of Line time-stepping towards steady state. I did uniform grid first and then a non-uniform grid later. Compared to the analytical solution. The uniform grid results are right but not the non-uniform one. It seems the upper boundary condition is wrong when using non-uniform grid.

```
# grid set up
L = 50.0 # domain [0,L]
N = 1000 # number of grid points
# a uniform grid
h = L/(N+1) # spatial steps
x = collect(range(h, step=h, length=N))
dx = vcat(x,L).-vcat(0,x)
# a non-uniform grid (finer mesh at the boundaries)
x = L*sin.(collect(range(h, step=h, length=N))/L*π/2).^2
dx = vcat(x,L).-vcat(0,x)
# a simple diffusion equation
D=10.0
Δ2 = CenteredDifference(2, 2,dx, N);
bc = RobinBC((0,-D,10.0),(1.0,0.0,0.0),dx,1);
function du_operator(du,u,parm,t)
du .= D.*(Δ2*bc*u) .+1
end
prob_operator = ODEProblem(du_operator,zeros(N),(0.0,10e3));
cb = TerminateSteadyState(1e-12, 1e-9,DiffEqCallbacks.allDerivPass);
@time sol = solve(prob_operator,CVODE_BDF(linear_solver=:Band,jac_upper=1,jac_lower=1),
reltol=1e-9,abstol=1e-12,callback=cb,tstops=10.0);
# analytical solution
analytical_sol = 175 .- x .- x.^2/20
```