Sometimes, shapes from child cells were propagated into
parent cells in the L2N and LVSDB output.
Because of this fix, many testdata files have to be updated.
Fake pins: pins that happen because something connects to a cell at an
unexpected position. Such a pin is difficult to find. The solution is
to keep those nets and nur purge them so these nets can be identified
in the layout.
Here: is_floating? will be true only if there are no pins. Hence
nets with pins are not removed. is_passive is introduced for nets -
passive nets are such that don't have elements, but a pin.
Circuits are purged if they only have passive nets.