Feature finding in 3d

See https://github.com/soft-matter/trackpy/issues/161 for motivation

- Allows tuple values in for diameter, separation and smoothing_size in locate routine. If an integer is given, it is converted to a tuple with the right dimension. Order is the same as the image dimension order (z,y,x normally)
Ah, I see below that `radius` will definitely be an integer if we enter `local_maxima` via `locate`. If the user calls `local_maxima` directly, they can get themselves into trouble with non-integer `radius`, but I'm not sure that it's worth trying to protect them from themselves. `locate` needs to be as user-friendly as possible, but `local_maxima` is down in the weeds. Agree?