Update units.py

Author: sici17

package/MDAnalysis/units.py

@@ -47,6 +47,7 @@
    charge       :math:`e`      :math:`1.602176565 \times 10^{-19}` As
    force        kJ/(mol·Å)     :math:`1.66053892103219 \times 10^{-11}` J/m
    speed        Å/ps           :math:`100` m/s
+   mass         u              :math:`1.66053892103219 \times 10^{-27}` kg
    =========== ============== ===============================================
 
 Implementation notes
@@ -407,6 +408,7 @@ def __getitem__(self, key):
     "charge": "e",
     "force": "kJ/(mol*A)",
     "speed": "A/ps",
+    "mass": "u",
 }