Feature request: limit number of shapes produced by net tracer

[klayoutmatthias]

For details see this forum discussion: https://www.klayout.de/forum/discussion/1435/tracing-of-large-power-nets#latest

[klayoutmatthias]

The implementation now looks like this:

There is one small topic I can't implement so far: the collected number of shapes may be a little less (usually by 1) than the specified depth. This is due to internal seed shapes which are collected but not reported. I think this is not a practical limitation.