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)
This line is confusing. Should it be `if np.any(np.greater(separation, 0))`? I think this is technically correct because `True > 0`, but the second `>` is superfluous.