Fix dg facetintegrator

[PaulSt]

Hello everyone!

the first commit fixes a memory error on DG facet terms that appears on facets between quads and trigs, or mixed spaces. Example:

from ngsolve import *
from netgen.occ import *

mesh = Mesh(OCCGeometry(Circle((0,0),1).Face(),dim=2).GenerateMesh(maxh=1,quad_dominated=True)).Curve(5)
fes = L2(mesh, order=5, dgjumps=True)
u,v = fes.TnT()
n = specialcf.normal(2)
a = BilinearForm(n*(grad(u)+grad(u.Other()))*(v-v.Other())*dx(skeleton=True)).Assemble()
print(a.mat)
print(sum(a.mat.AsVector()))

gives NaN (sometimes)

Second commit makes sure mixed spaces survive in the DG linearized facet matrix.