Reading a Gromacs topology fails with insertion code or negative numb…

…er (#1225)

* reading gromacs topologies with insertion codes fixed

* using conditional instead of try-except clause

parmed/gromacs/gromacstop.py

@@ -583,8 +583,9 @@ def _parse_atoms(self, line, params):
         else:
             atom = Atom(atomic_number=atomic_number, name=words[4],
                         type=words[1], charge=charge, mass=mass)
-        # check for insertion code
-        if words[2].isnumeric():
+
+        # check for insertion code and negative res number
+        if words[2].isnumeric() or (words[2].startswith('-') and words[2][1:].isnumeric()):
             icode = ''
             resnum = int(words[2])
         else: