[surface] Set the fitting region of pcl::on_nurbs::FittingSurface

[ZhuLingfeng1993]

When I use pcl::on_nurbs::FittingSurface to fitting a point cloud, I want to the fitting result fill a certain bounding box region on xy plane.

Currently, I use pcl::on_nurbs::FittingSurface::initNurbs4Corners to set an initial fitting region that larger than the expected bbox region, but the fitting result sometimes may not fill the bbox region, that means there is no fitting result in some part of the expectd bbox region.

So I hope to add a feature to set the fitting region of pcl::on_nurbs::FittingSurface.