atom type names in Masses section of DATA file are consistent

Atom type names are now listed as comments in the Masses section of the DATA files generated by moltemplate for all popular force fields supported by moltemplate. (The atom type names will now be consistent with the string following "@atom:") The scripts that convert these force field files into LT format have been updated for python3 compatibility. The "cleanup_moltemplate.sh" script has been updated to preserve these atom type names (and not erase them). Finally, the "ltemplify.py" script now has a "-preamble" argument allowing users to prepend text at the beginning of the files generated by "ltemplify.py".