remove stubs, fix several pcl-related items

My take at addressing #428.

The stubs can be fixed, but I find them to be brittle. We now control all PCL inclusion with PDAL_HAVE_PCL (before it was a mix of PDAL_HAVE_PCL and WITH_PCL, sometimes conditioned on WITH_STUBS).

Now we just need to make sure to clearly document that PCLBlock support is only available through my fork.