start grading implementation

common/lib/capa/capa/chem/chemtools.py

-def vsepr_grader():
-    pass
+from collections import OrderedDict
+
+
+def vsepr_build_correct_answer(geometry, atoms):
+    correct_answer = OrderedDict()
+    correct_answer['geometry'] = geometry
+    correct_answer['atoms'] = OrderedDict(atoms)
+    return correct_answer
+
+
+def vsepr_grade(user_input, correct_answer):
+    if user_input['geometry'] != correct_answer['geometry']:
+        return False
+    if user_input['atoms'].values() != correct_answer['atoms'].values():
+        return False
+    return True
+