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)
They should all be greater than 0, else we get a division by zero 3 lines below. So all separation should be nonzero. The `> 0` was a leftover from the previous version, that will be deleted.