add symmetric iteration routine, related to #5

tests/test_iteration.py

@@ -0,0 +1,25 @@
+from dopt_basics import iteration
+
+
+def test_symmetric_iter_1():
+    l1_in = [1, 2, 3, 4, 5, 6]
+    l1_out = [1, 6, 2, 5, 3, 4]
+
+    l1_calc = list(iteration.symmetric_iter(l1_in))
+    assert len(l1_in) == len(l1_calc)
+    assert len(l1_out) == len(l1_calc)
+
+    for truth, calc in zip(l1_out, l1_calc):
+        assert truth == calc
+
+
+def test_symmetric_iter_2():
+    l1_in = [1, 2, 3, 4, 5]
+    l1_out = [1, 5, 2, 4, 3]
+
+    l1_calc = list(iteration.symmetric_iter(l1_in))
+    assert len(l1_in) == len(l1_calc)
+    assert len(l1_out) == len(l1_calc)
+
+    for truth, calc in zip(l1_out, l1_calc):
+        assert truth == calc