Reading a Gromacs topology fails when an insertion code exists

[Valdes-Tresanco-MS]

Hi
First of all, parmed is awesome. Good work!

Recently, I have used parmed to read an antibody Gromacs topology. The topology contains insert codes and when reading it ends in an error.
ValueError: invalid literal for int () with base 10: '27A'

When the line that contains the information of each atom is processed, it is divided by spaces, so if the residue contains an insertion code then this error is obtained.
A possible solution would be:
change this method https://github.com/ParmEd/ParmEd/master/parmed/gromacs/gromacstop.py#L562-588

def _parse_atoms(self, line, params):
    """ Parses an atom line. Returns an Atom, resname, resnum """
    words = line.split()
    try:
        attype = params.atom_types[words[1]]
    except KeyError:
        attype = None
    if len(words) < 8:
        if attype is not None:
            mass = attype.mass
            atomic_number = attype.atomic_number
        else:
            mass = -1
            atomic_number = -1
    else:
        mass = float(words[7])
        if attype is not None and attype.atomic_number >= 0:
            atomic_number = attype.atomic_number
        else:
            atomic_number = AtomicNum[element_by_mass(mass)]
    charge = float(words[6]) if len(words) > 6 else None
    if atomic_number == 0:
        atom = ExtraPoint(name=words[4], type=words[1], charge=charge)
    else:
        atom = Atom(atomic_number=atomic_number, name=words[4],
                    type=words[1], charge=charge, mass=mass)
    # fix this error
    if words[2][-1] in letters:
        icode = words[2][-1]
        resnum = int(words[2][:-1])
    else:
        icode = ''
        resnum = int(words[2])
    return atom, words[3], resnum, icode        

and modified this line https://github.com/ParmEd/ParmEd/master/parmed/gromacs/gromacstop.py#L407

elif current_section == 'atoms':
      atom, resname, resnum, icode = self._parse_atoms(line, params)
      molecule.add_atom(atom, resname, resnum, inscode=icode)

I do not do a PR because it can take time and it is simple. I hope this helps and can fix it.
Best regards
Mario

[Valdes-Tresanco-MS]

Fixed via #1219 and #1220