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)
@nkeim is right, I added a comment now.
The broadcasting actually already does the job, so it's now `results[:, :mass_index]/separation`