Bug fixes for GB.  Now using strength parameter mixing ideas from Wu
et al. [J. Chem. Phys. 135, 155104 (2011)].  This helps get the
dissimilar particle mixing behavior to be the same whichever order the
two particles come in.  This does require that the force field file to
specify explicitly the values for epsilon in the cross (X), side-by-side (S), 
and end-to-end (E) configurations.