Changes following @danielballan remarks

- removed unused `ndim` variable from new `percentile_threshold()`
- updated eliminate duplicates to tuple separation, missed it after
rebasing after PR https://github.com/soft-matter/trackpy/pull/143
- allow tuple value for `noise_size` / `lshort`
- rewrote `diameter`, `separation`, `smoothing_size`, `noise_size`
initiation
- some minor adaptations