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)
Nice! I have been told that this approach -- one dimension at a time -- is paradoxically *faster* that an N-dimensional filter. So we may actually get an accidental performance boost here. I want to do some experimenting with various bandpass implementations to get a better handle on this.