Merge pull request #2005 from peastman/ssbond

Regenerated files to fix missing external bonds

wrappers/python/simtk/openmm/app/data/amber14/protein.ff14SB.xml

 <ForceField>
   <Info>
-    <DateGenerated>2017-11-25</DateGenerated>
-    <Source Source="leaprc.protein.ff14SB" md5hash="2fbe80b999846068f2a1b6f3c50fcbb9" sourcePackage="AmberTools" sourcePackageVersion="17">leaprc.protein.ff14SB</Source>
+    <DateGenerated>2018-03-02</DateGenerated>
+    <Source Source="leaprc.protein.ff14SB" md5hash="2fbe80b999846068f2a1b6f3c50fcbb9" sourcePackage="AmberTools" sourcePackageVersion="17.6">leaprc.protein.ff14SB</Source>
     <Reference>Maier, J.A., Martinez, C., Kasavajhala, K., Wickstrom, L., Hauser, K.E., and Simmerling, C. (2015). ff14SB: Improving the Accuracy of Protein Side Chain and Backbone Parameters from ff99SB. J. Chem. Theory Comput. 11, 3696-3713.</Reference>
   </Info>
   <AtomTypes>
@@ -270,9 +270,9 @@
       <Bond atomName1="CB" atomName2="HB3"/>
       <Bond atomName1="CB" atomName2="SG"/>
       <Bond atomName1="C" atomName2="O"/>
+      <ExternalBond atomName="SG"/>
       <ExternalBond atomName="N"/>
       <ExternalBond atomName="C"/>
-      <ExternalBond atomName="SG"/>
     </Residue>
     <Residue name="GLH">
       <Atom charge="-0.4157" name="N" type="protein-N"/>
@@ -1207,6 +1207,7 @@
       <Bond atomName1="CB" atomName2="SG"/>
       <Bond atomName1="C" atomName2="O"/>
       <Bond atomName1="C" atomName2="OXT"/>
+      <ExternalBond atomName="SG"/>
       <ExternalBond atomName="N"/>
     </Residue>
     <Residue name="CGLN">
@@ -2128,6 +2129,7 @@
       <Bond atomName1="CB" atomName2="HB3"/>
       <Bond atomName1="CB" atomName2="SG"/>
       <Bond atomName1="C" atomName2="O"/>
+      <ExternalBond atomName="SG"/>
       <ExternalBond atomName="C"/>
     </Residue>
     <Residue name="NGLN">