feature request: create a def single/mulitpart path import option

[stefanottili]

If there are multiple path segments on the same layer, klayout currently creates multiple two point path segments.
It used to write these as a single multi-point path.
Please expose the single vs multipart path behavior as an def import option.

src/plugins/streamers/lefdef/db_plugin/dbDEFImporter.cc

427 #if 0
428 // single path
...
435 #else
436 // multipart paths

[klayoutmatthias]

The option is called "--lefdef-joined-paths" for strm* buddy tools
and "joined_paths" for pya/RBA.

Wires with multiple segments are combined into long paths when
possible - this fails on anisotropic widths and 45 degree segments.