Implement new function create_nodal_elements

This function can be used to create elements from node set defined
in mesh. Resulting elements are of type Poi1, that is, they don't
have any volume but it's still possible to add boundary conditions
to them which are then enforced in discrete sense.

test/test_create_nodal_elements.jl

@@ -0,0 +1,16 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+using Base.Test
+
+using JuliaFEM
+using JuliaFEM.Preprocess
+
+@testset "test create nodal elements" begin
+    m = Mesh()
+    add_node!(m, 1, [0.0, 0.0])
+    add_node_to_node_set!(m, :test, 1)
+    els = create_nodal_elements(m, "test")
+    fel = first(els)
+    @test fel.connectivity == [1]
+end