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)
One could choose to only noise filter in x,y because feature sizes in z tend to be small. In that case subpx accuracy is not reliable. But anyway this was my quick solution to this problem.  I do agree that we should just raise an error, else the user doesn't know what is happening.

We could also allow a tuple value for noisefilter, what do you think?