glycam2gromacs.py: Avoid conflicts with general Amber forcefield

Don't redefine atomtypes already defined in Amber. Users are expected to
always include Amber forcefield before Glycam.

Filter out unsupported atomtypes (starting with number, e.g., 2C and
3C). Amber forcefield 12SB is not ported to Gromacs at the moment and
Gromacs requires atom types to start with a letter.

Filter out water atomtypes and its bonded parameters. Use water from
general Amber forcefield.

Filter out a dihedral with undefined atomtype 'NH'.

Add option to prefix atomtypes with 'G_' and avoid renaming atomtypes
from general Amber forcefield.