Add overload for LagrangeBasis with single node

This MR provides an overload to LagrangePreBasisCreatorImpl to return a single-node prebasis lagrange<k>() when using LagrangeBasis<Grid, k>.

examples/init/boundary.dat.2d

@@ -8,7 +8,7 @@ ellipt->solver->relative tolerance: 1.e-10
 ellipt->solver->info:           1
 ellipt->solver->precon:         ilu
 
-ellipt->output[]->format:      vtk
-ellipt->output[]->filename:    boundary.2d
-ellipt->output[]->name:        u
-ellipt->output[]->output directory: ./output
+ellipt->output->format:      vtk
+ellipt->output->filename:    boundary.2d
+ellipt->output->name:        u
+ellipt->output->output directory: ./output