Decrease space between vertical beams by length-fraction.

Decrease space between vertical beams by length-fraction.

This causes 16th and higher grace beams to be closer together.

Thanks to Daniel Spreadbury for noting this in his blog,
http://blog.steinberg.net/2015/03/development-diary-part-10/

To fix the regression in grace-start.ly, decrease collision-padding
for grace beams: since grace beams are smaller, objects must come
closer to be considered a collision.

To fix regressions where grace stems become too large, decrease
importance of forbidden quant scoring if length-fraction != 1.0..