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)
Once the particle coordinates are in cKDTree they're not coming back out. So if you rescale each axis as you build the tree, query_pairs(1) will work. Casper, am I right that that's how the code works? If so, a comment is probably in order.