Reduced share memory for CustomGBForce

Here are my latest benchmarks:

OBC2: 314
OBC1: 280
GBn: 220

So the cost of using CustomGBForce is now down to only about 10%.  GBn is about 30% slower than the handwritten OBC, or 20% slower than the CustomGBForce version.  And twice as fast as it was before I started on optimizations!