about 1-4 interactions

two implement methods:

1. a list of [pairtype] in ffnonbond.itp, gen_pair=no, only useful for force field like Gromos.

2. For OPLS and Amber, 1-4 interactions are scaled by a factor, so theirs is no [pairtype] in ffnonbond.itp, and gen_pair=yes. It should be noted that the coulomb interaction is also scaled.

exclusion=3 is default for all force fields, because [pairs] are included in all force fields.