updated GAFF2, amber2lt.py, and a few alkane examples

The @atom:hw and @atom:ow atom types in "gaff2.lt" now have been given epsilon and sigma parameters (of 0.0 and 1.0). These parameters are not correct, but at least users who don't need these atom types and want to use "gaff2.lt" will be able to do so (without LAMMPS complaining about missing pair_coeffs).

Similarly the amber2lt.py program will now supply default epsilon and sigma parameters (of 0.0 and 1.0) for atoms when this information is lacking in the FRCMOD or DAT file containing the force-field parameters (such as "gaff2.dat").

The amber2lt.py program accepts a slightly wider range of FRCMOD file formats.

Thanks to Karteek Bejegam and Jonathan Campeggio for their debugging help and suggestions!