DRC "Source#select" does not work as documented

[klayoutmatthias]

1.) Contrary to the documentation, the "select" feature modifies the source rather than returning a copy.

Technical reason is the "dup" function which does not create a deep copy.

Workaround: Instead of using

only_CELL_and_below = source.select("-TOP", "+CELL")

use

only_CELL_and_below = layout(source.cell_obj).select("-TOP", "+CELL")

2.) There is an undocumented and useful feature to use only "-" for disabling the top cell. But it's not working :(

To disable to the top cell, use

source.select("-" + source.cell_name, "+CELL")

3.) Hierarchical selection "with cells below" may not render the desired results in deep mode. Reason: this requires cell variants as one cell may be selected in one path, but not in the other. Currently, cell variant building is not supported.

[klayoutmatthias]

PR #654 fixes the three issues mentioned in the ticket.