Added experimental parallel IBBA. Experiments show speedups ranging from 4x-10x depending on the problem (on fractus with 11 worker threads). Short running problems experience a slight slow down relative to the serial implementation, but usually returns tighter bounds.
Added experimental parallel IBBA. Experiments show speedups ranging from 4x-10x depending on the problem (on fractus with 11 worker threads). Short running problems experience a slight slow down relative to the serial implementation, but usually returns tighter bounds.

soarlab/gelpia,soarlab/gelpia,soarlab/gelpia,soarlab/gelpia