Switch to OpenMP for multithreading.

-- RMonica
Change weight computation class.

The old ComputeWeight method is now renamed into ComputeWeightForCloud(clouds,cloud_idx,set,set_weight)
and receives a new Real parameter.

The new ComputeWeight method computes the weight of the set regardless of the current point cloud,
that weight becomes the new parameter for ComputeWeightForCloud.

-- RMonica