mostly production-level updates to fencegui

- core_int: switch to GIMP projection
- fencegui: various GUI improvements, simplifications for accelerated
use, better pre-determined transect intersections, added GIMP projection
- mergegui: added GIMP projection, SNR analysis
- pickgui: added GIMP projection
long-term updates to GUIs

- removed flattening from FENCEGUI, squashed various bugs
- added option to display layer colors in terms of age to MERGEGUI (if
available)
- added a field to pk that is included in PICKGUI to ensure that layer
numbers do not get repeated
- improved error-checking for INTERSECTI