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)
Indeed, a tuple value for `lshort` input, or `noise_size` as it is named in `locate()`. I will do that. The `xrange` will be replaced anyway by the code you proposed (`np.any(2 * lshort > llong)`), but thanks for mentioning because I used it too in another PR for pims.