Generalize errornorm computation.

[jorgensd]

Can use ufl.reconstruct to increase the element degree:

def error_L2(uh, u_ex, degree_raise=3):
    # Create higher order function space
    ufl_el = uh.function_space.ufl_element()
    new_el = ufl_el.reconstruct(degree=degree_raise+ufl_el.degree())
    mesh = uh.function_space.mesh
    W = fem.FunctionSpace(mesh, new_el)
    # Interpolate approximate solution
    u_W = fem.Function(W)
    u_W.interpolate(uh)

    # Interpolate exact solution, special handling if exact solution
    # is a ufl expression or a python lambda function
    u_ex_W = fem.Function(W)
    if isinstance(u_ex, ufl.core.expr.Expr):
        u_expr = fem.Expression(u_ex, W.element.interpolation_points())
        u_ex_W.interpolate(u_expr)
    else:
        u_ex_W.interpolate(u_ex)

    # Compute the error in the higher order function space
    e_W = fem.Function(W)
    e_W.x.array[:] = u_W.x.array - u_ex_W.x.array

    # Integrate the error
    error = fem.form(ufl.inner(e_W, e_W) * ufl.dx)
    error_local = fem.assemble_scalar(error)
    error_global = mesh.comm.allreduce(error_local, op=MPI.SUM)
    return np.sqrt(error_global)