Add navier-stoke tests mimicing pdelab-howto

test/navier-stokes/navierstokes_howto_2d_dg_quadrilateral.mini

+__name = navierstokes_howto_2d_dg_quadrilateral_{__exec_suffix}
+__exec_suffix = symdiff, numdiff | expand num
+
+cells = 8 8
+extension = 1. 1.
+
+printmatrix = true
+printresidual = false
+
+[wrapper.vtkcompare]
+name = {__name}
+extension = vtu
+
+[formcompiler]
+numerical_jacobian = 0, 1 | expand num
+# compare_l2errorsquared = 1e-8

test/navier-stokes/navierstokes_howto_2d_dg_quadrilateral.ufl

+# This example mimics the PDE of the pde-howto (after some
+# adaptions). It can be used to compare residual evaluations and
+# setting up the jacobi-matrix between the generated code and the
+# pdelab operator.
+
+cell = quadrilateral
+
+P2 = VectorElement("DG", cell, 2)
+P1 = FiniteElement("DG", cell, 1)
+TH = P2 * P1
+
+v, q = TestFunctions(TH)
+u, p = TrialFunctions(TH)
+
+x = SpatialCoordinate(cell)
+g_v = as_vector((4*x[1]*(1.-x[1]), 0.0))
+
+# bctype = conditional(x[0] < 1. - 1e-8, 1, 0)
+# ds = ds(subdomain_id=1, subdomain_data=bctype)
+
+n = FacetNormal(cell)('+')
+eps = -1.0
+sigma = 1.0
+
+rho = 1.0
+mu = 1.0
+
+h_e = Min(CellVolume(cell)('+'), CellVolume(cell)('-')) / FacetArea(cell)
+
+r = mu * inner(grad(u), grad(v))*dx \
+  - p*div(v)*dx \
+  - q*div(u)*dx \
+  + rho * inner(grad(u)*u,v)*dx \
+  + mu * inner(avg(grad(u))*n, jump(v))*dS \
+  + mu / h_e * sigma * inner(jump(u), jump(v))*dS \
+  - mu * eps * inner(avg(grad(v))*n, jump(u))*dS \
+  - avg(p)*inner(jump(v), n)*dS \
+  - avg(q)*inner(jump(u), n)*dS \
+  - mu * inner(grad(u)*n, v)*ds \
+  + mu / h_e * sigma * inner(u-g_v, v)*ds \
+  + mu * eps * inner(grad(v)*n, u-g_v)*ds \
+  + p*inner(v, n)*ds \
+  + q*inner(u-g_v, n)*ds
+
+forms = [r]
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