Added the option to filter input point cloud by rectangle (extent) and expression in the following PDAL algorithms:

• boundary
• clip
• density
• export to raster (normal and TIN variants)
• export to vector
• merge
• thin

This allows the processing of only a subset of points from the input file(s), without the need to performing an intermediate filtering step or generating temporary files.
For these who need only filtering there is also a separate Filter algorithm capable of filtering either by extent, by expression or by their combination.