Modified lrowac2isis documentation, added PDS definition to Glossary #Fixes 3886

            non-valid pixels.

          </definition>

	  <definition name="Planetary Data System">

            The Planetary Data System (PDS) is an archive of scientific data acquired 

	    from NASA planetary missions, astronomical observations, and laboratory 

	    measurements. By developing standards for data and archive formats and 

	    architectures for use within the archive, the PDS ensures long-term access 

	    and usability of NASA planetary data.

	    <p>

	      The PDS is organized as a federation of ten teams (nodes) geographically 

	      distributed throughout the United States. Science disciplines represented 

	      by the PDS include the Atmospheres Node, Geosciences Node, Cartography and 

	      Imaging Sciences Node, Planetary Plasma Interactions Node, Ring-Moon Systems 

	      Node, and Small Bodies Node. Two support entities include the Engineering Node 

	      and the Navigation and Ancillary Information Facility (NAIF) Node. Additional 

	      functions include a Radio Science specialist and a User-Centered Design team.

	    </p>

          </definition>

          <definition name="NULL">

            NULL is a Special Pixel value that is tracked and reported to the

            user.  NULL represents "No Data".   NULL is set in all 8, 16, 32-bit