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)
It's better to test how an input *behaves* rather than what type it *is*. In place of `if type(diameter) == int` you can use `if not isinstance(diameter, collections.Iterable)`. There are many datatypes we could accept here -- list, tuple, ndarray, future successors to ndarray, etc. -- and we don't want to special-case them all.

Also, PEP8-wise, we avoid one-line if statements.