Feature/depth line model

***  THIS IS NOW OBSOLETE ******

All was incorporated into the multi-freq-plot PR.

This adds a depth_line model and interface and makes several edit pane format changes.  This will run and most of the new formatting here works though the depth_line object is not fully integrated so we cannot yet make new lines anymore.  
Major changes are coming to the edit pane structure in order to have the multiple freq view with the slice view to the right as we discussed.  There is a test model working but this reformatting will be put into another branch.