CMake: Fix building with separate build dir

When building the python SWIG bindings in a separate build dir,
the build needs the directory of the build include path to get
pdal/pdal_defines.h.
Merge pull request #254 from simonsonc/separate-build-dir

CMake: Fix building with separate build dir
Implement Python 3 support #256
Add 'pdal diff' utility #255
numPointsNeeded in PCLBlock should be updated by the number of points read, not the number remaining after filtering
Support reading PointBuffers that have no XYZ on them #123
Remove supportsIterator().  Never called.
Merge pull request #257 from PDAL/no-supports-iter

Remove supportsIterator().  Never called.
Add: splitter filter to generate spatially contiguous point buffers of fixed size
Merge pull request #258 from PDAL/filter-create-tiles-alt2

Add: splitter filter to generate spatially contiguous point buffers of fixed size
Begin using 'option' rather than 'set' for PDAL options

Options become slightly more concise (drops the 'CACHE BOOL') and
enables use of CMAKE_DEPENDENT_OPTION macro, which could be useful
for the GDAL/GeoTIFF/TIFF trio among others.
small cleanups to filters.tiler
use make generator by default
Merge remote-tracking branch 'origin/use-options-command'
Fix typo.
Merge pull request #263 from PDAL/i262

Fix typo.
fix #261, only link libxml2 if WITH_LIBXML2 is TRUE
Merge pull request #265 from PDAL/issues/261-guard-linking-libxml2

fix #261, only link libxml2 if WITH_LIBXML2 is TRUE
Don't use stage in iterator.
Add NullPointBuffer.
Modify PointBuffer to behave correctly when used as a NullPointBuffer.