Fixing bad merge and updating higher order test

Author: Akash Garg

src/MOLFiniteDifference/MOL_discretization.jl

@@ -195,7 +195,7 @@ function SciMLBase.symbolic_discretize(pdesys::ModelingToolkit.PDESystem,discret
         # Use max and min to apply buffers
         central_neighbor_idxs(II,j) = stencil(j) .+ max(Imin,min(II,Imax))
         central_neighbor_space(II,j) = vec(grid[j][map(i->i[j],central_neighbor_idxs(II,j))])
-        central_weights(d_order, II,j) = DiffEqOperators.calculate_weights(2, grid[j][II[j]], central_neighbor_space(II,j))
+        central_weights(d_order, II,j) = DiffEqOperators.calculate_weights(d_order, grid[j][II[j]], central_neighbor_space(II,j))
         central_deriv(d_order, II,j,k) = dot(central_weights(d_order, II,j),depvarsdisc[k][central_neighbor_idxs(II,j)])
 
         # get a sorted list derivative order such that highest order is first. This is useful when substituting rules

test/MOL/MOL_1D_HigherOrder.jl

@@ -103,11 +103,14 @@ end
     # Discretization
     dx = 0.4; dt = 0.2
 
-    discretization = MOLFiniteDifference([x=>dx],t;centered_order=4)
+    discretization = MOLFiniteDifference([x=>dx],t;centered_order=4,grid_align=center_align)
     pdesys = PDESystem(eq,bcs,domains,[x,t],[u(x,t)])
     prob = discretize(pdesys,discretization)
 
     sol = solve(prob,Tsit5(),saveat=0.1,dt=dt)
+
+    @test sol.retcode == :Success
+
     xs = domains[1].domain.lower+dx+dx:dx:domains[1].domain.upper-dx-dx
     ts = sol.t