updates in scenario.py:
- save files have .json extension
- if path to save dir doesn't exist, it will get created but only one
  level deep. See comments in __init__ for more details
- save() method now clobbers all files in savelocation. So save()
  will only store datafiles and json associated with current model
  being saved

test_persist.py
- cleaned up the tests - removed finalizer to delete save_model after
  each test.
- added more tests for checking exceptions raised by Scenario class
fixes to cats3d and tidecur so topology files load in gui gnome; fixed bug in dagtreeio
updated docs and removed ubuntu_requirements.txt since it is redundant.
Only need one requirements.txt
Merge branch 'master' of https://trac.orr.noaa.gov/git/GNOME2
Merge branch 'master' of https://trac.orr.noaa.gov/git/GNOME2
fixes for gui GNOME so netcdf topology files load; also fix for tidecur topology from a location file resource
Merge branch 'master' of https://trac.orr.noaa.gov/git/GNOME2
Merge branch 'master' of https://trac.orr.noaa.gov/git/GNOME2