STYLE: debug output in tract_flip_endpoints_in_label #151

tract_flip_endpoints_in_label (tract_querier/tract_math/operations.py ~822 on master) prints image_data.sum() to standard output, which looks like leftover debugging output; it mixes with the operation's own output. (The other print in the function, Flipped N tracts, is informative.) #143 fixes the crashes in this operation but leaves the print.