add packing functions for Instances and KCells

[sebastian-goeldi]

example.py

```python import kfactory as kf from kfactory.packing import pack_instances, pack_kcells @kf.cell def box(width: int, height: int) -> kf.KCell: c = kf.KCell() c.shapes(c.kcl.layer(1, 0)).insert(kf.kdb.Box(width, height)) return c c = kf.KCell("Box Packing") ig1 = pack_kcells( c, [box(w, h) for w, h in [(500, 500) for _ in range(15)] + [(15_000, 1000)]], max_width=16000, spacing=1000, ) small_boxes = [c << box(w, h) for w, h in ((500, 500) for _ in range(15))] bigbox = c << box(15_000, 1000) bigbox.transform(kf.kdb.Trans.R90) ig2 = pack_instances( c, small_boxes + [bigbox], max_width=16000, spacing=1000, ) ig2.movex(ig1.xmax + 5000 - ig2.xmin) c.show() ```

gives:

Also clean up some of the instance group code.

@joamatab @akshat-ja @gdspaul

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[akshat-ja]

Let me test a bit more on my side @sebastian-goeldi then I'll green light.