[pdal] Raster overlay support added

[Howard Butler]

All,

I have added a filter, incorrectly called Colorization, that supports querying GDAL to fetch raster values coincident with point data. While it is usually the case that you want to fill in the Red, Green, and Blue dimensions, you can also use this to fetch any other dimensions as well.  The default options assume that you are fetching color information.  The features of this filter currently include the ability to select the GDALRasterBand->Dimension mapping as well as individually apply a scaling factor to the returned raster results before stuffing them into your pdal::Dimension.

I will probably rename this filter to something like RasterOverlay or some other more appropriate name after discussing things at the sprint.

Howard