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)
I see that we have to check `lshort` against `llong` in each dimension. But why adjust `lshort` instead of raising an error as before? I think I'd prefer

    if np.any(2 * lshort > llong):
        # raise ValueError as before

Am I misunderstanding your reasoning?