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 think you should make `range` `np.arange` here.` Plain old `range` returns a generator in Python 3, which I think will not satisfy `meshgrid` below.