Calculate level of nestedness of stream gages.

Some applications need to know if a stream gage is upstream of other stream gages for the purposes of working through the network in order of nesting level. e.g. calibrate parts of a model for headwaters only, then the regions just downstream of headwaters, then the next down, and so on. To accomplish this, an ordering of stream gages indicating their level of nesting is needed.

This level of nesting is actually very similar to stream order. Stream gages with nothing upstream get 1, stream gages with only 1s upstream get 2, stream gages with 2s upstream get 3, and so on.

The difference here is that, inlike stream order, "gage order" needs to be one more than the greatest upstream neighbor. This will require a tweak to the stream order logic but is much the same architecturally.

Start by locating the existing stream-order logic and its related entry points. Implement the analogous gage-order calculation so headwater gages are level 1 and each other gage is one greater than its greatest upstream neighbor; done when the ordering supports downstream calibration sequencing.

Written by the indexing model from the issue text.