Take the 1-4 non-bonded scaling parameters from the Amber topology files if
present, defaulting to 1.2 (SCEE) and 2.0 (SCNB) if they're not present. If
values for scee and scnb are provided to readAmberSystem they will override any
values in the topology file, but a warning will be printed (since ignoring the
scaling parameters in the topology file really changes the force field).