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)
Ohhh I see. Clever. Yes, please add a comment.

I wonder if we need to build out that tiled array (which is a little expensive) or if broadcasting can do this for us.. Try `.div` instead of `/`.