Add "tif" to p2g docs

We can export tifs, so we should mention that in the docs.
add ::flush() method to LAS writer so that LASzip stuff gets written in NITF writer #503
add LAZ-writing NITF example
Merge pull request #503 from PDAL/issues/503-laszip-flush

LAS Writer needs to flush the LASzip stuff
Merge pull request #502 from gadomski/mention-tif-in-p2g-docs

Add "tif" to p2g docs as an available output format