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)
OK, I think we should allow a tuple for the `lshort` and `llong` inputs. Is that what you mean by "a tuple value for noisefilter"?

Also, minor fix I just caught: `xrange` breaks in Python 3. We could use `six` to work around that, but the advantage of `xrange` here is negligible, so I think you can use `range` instead.