Support GDSII Output for Place and Route Rules

[QuantamHD]

We should use the magic instance we already built to stream the output DEF files into GDSII

[lpawelcz]

Hi @QuantamHD, I would like to work on this issue. Should I extend _place_and_route_impl to perform additional steps or add a second rule, something like asic_flow_finish?
I would also like to add other missing steps which are done in OpenROAD-flow-scripts:

• metal fill
• antenna check
• parasitics extraction
• and finally, writing GDS

As for the tool for writing GDS files I would prefer to use KLayout (actually only KLayout python library, which does not depend on Qt) to make the flow similar to OpenROAD-flow-scripts.
For that I should also work on #20

[QuantamHD]

Thanks for working on this! Could we start with creating it as a separate rule? Inside Google we have a different dependency manager that may or may not play well with the Klayout python module. I'd like to get that working without blocking a merge in this repo.

Once we have it working it shouldn't be a big lift to get it merged together. Thanks again! :)