Provide implicit LVS connections for double-height std cells

[klayoutmatthias]

This is related to this thread: https://www.klayout.de/forum/discussion/1384/lvs-for-double-height-standard-cell#latest

Essentially the request is to support implicit connections for non-top-cells too.

[klayoutmatthias]

This patch provides a new version of "connect_implicit" which takes a cell selector and applies the implicit connections to the selected cells (which can be child cells)