clipping, filling masked region

[matthewhanson]

If the site file goes outside the collected data region those values get filled with nonsense (see current dems in polygon 003).

Best solution is to use density as a mask, however it should use the complete density image as a mask, because the dtm and dsm density versions could have missing values but were properly filled during the gapfill process.

So, use whole density image as a mask, but it may or may not exist as it currently is not a prerequisite to generate dems.

For performance reasons, it would be best to add the mask during or at the end of the gap-filling process.