Implemented ImageZigzagPersistence * Changed ZigzagPersistence to support a visitor, and implemented ImageZigzagPersistence * examples/rips/rips-zigzag now computes using ImageZigzagPersistence * PersistenceDiagram no longer records zero persistence pairs * Added utilities/memory.h with report_memory() function

add_subdirectory			(geometry)
add_subdirectory			(utilities)