Simplify API for CPW ports (single port with geometry)

[mdmaas]

In QPDK and possibly other PDKs, the user defines a single port on a CPW line. However, the simulation needs to define two rectangles at the proper positions.

This PR modifies the existing API to match what comes from the user/PDK.

# Before (two ports per end):
sim.add_cpw_port("P2", "P1", layer="topmetal2", length=5.0)

# After (single port + geometry):
sim.add_cpw_port("o1", layer="topmetal2", s_width=20, gap_width=15, length=5.0)

Note that now, the user defines a single "o1" port on the CPW end, and then for the simulation, geometric parameters need to be set.

Future work: those parameters could be extracted from the PDK, if a cross-section is present. We had that in palace-server, and could eventually merge the functionality.

Also note that this PR is needed to support the QPDK #28.

[mdmaas]

Ouch, I made this PR on top of the local_sim branch.

[nikosavola]

I can approve if #13 is merged first or the common commits are dropped with rebase

[mdmaas]

Oh, good idea @nikosavola, let me try to rebase

[mdmaas]

There, it should be clean now.

[mdmaas]

Ok, this is passing now