Add test for ElementLineHermite

Add test to make sure that exceptions is thrown when using gdof with
index > 3.

tests/test_element_line_hermite.py

@@ -0,0 +1,14 @@
+from unittest import TestCase
+from skfem.element.element_line.element_line_hermite import ElementLineHermite
+
+
+class TestElementLineHermite(TestCase):
+
+    def test_index_error(self):
+        """ Tests that exception is thrown when accessing gdof with i > 3. """
+        element = ElementLineHermite()
+        with self.assertRaises(ValueError):
+            F = 0
+            w = 0
+            i = 4
+            element.gdof(F, w, i)