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)
The purpose of this `int()` was to make sure that if the user enters `diameter=5.` that it behaves that same as `diameter=5`. Numpy is usually smart about this -- `some_array[:4.]` is the same as `some_array[:4]` -- but if we remove this safeguard should add some tests. I think `diameter=5.` should work as expected but `diameter=5.5` should raise an informative error.