Add unit test for protonate.py

tests/test_protonate.py

@@ -0,0 +1,13 @@
+import propka.atom
+import propka.protonate
+
+
+def test_protonate_atom():
+    atom = propka.atom.Atom(
+        "HETATM 4479  V   VO4 A1578     -19.097  16.967   0.500  1.00 17.21           V  "
+    )
+    assert not atom.is_protonated
+    p = propka.protonate.Protonate()
+    p.protonate_atom(atom)
+    assert atom.is_protonated
+    assert atom.number_of_protons_to_add == 6