Antenna check: perimeter-only mode

[klayoutmatthias]

Apparently some PDKs ask for an Antenna check that entirely defines area in terms of perimeter (with a thickness value?).

The latest antenna rule check is providing "area only", "area with perimeter", but no "perimeter only" mode.

[klayoutmatthias]

The syntax of the implementation is:

errors = antenna_check(perimeter_only(gate, t), ...)

"t" is the "thickness". The perimeter is multiplied by t to render area units. The dimension of t is micrometers. "perimeter_only" can be applied to gate or metal or both.