depth in the crawling process where a
	page must be found in order for it to be processed. Resources found
	deeper than this level are not fetched and parsed by the spider. The value
	zero means unlimited depth.
	<p>
		The depth is calculated starting from the seeds, so, if a Spider scan
		starts with only a single URL (eg. URL manually specified), the depth is
		calculated from this one. However, if the scan starts with multiple
		seeds (eg. recurse and Sites tree node with children), a resource is process