fencegui fixes mostly

coreint - renamed to core_int.m
core_int - added Dye-3 core
fencegui - made it possible to unlink y/z axes of radargrams in 2D,
which makes it easier to match layers for radargram pairs that are
noticeably offset vertically
mergegui - adjusted how existing block files are found
pickgui - added additional NaN check for flattened surface and bed plots