Add Simulation/As-Built processing

[G]

The attached tar file contains a set of nine patches (generated with git format-patch) to add Simulation/As-Built processing to geda/gaf.

The README file in the examples/SAB directory contains details on how to use it and the motivation behind it..

Short version: The schematic components can have attributes attached. When processing the schematic with gnetlist these attributes can cause the netlist to be modified by discarding or bypassing components, or executing an external python script on the netlist. This occurs prior to the backend. So for example components which only exist for simulaton can be removed, e.g. spice directives, when generating a parts list or pcb netlist. Similarly components which have no role in the simulation, e.g. physical connectors, can be removed when generating a spice netlist.

There is quite a bit more but that is the quick version.

The patches are based on the current master branch and applied from the top of the geda/gaf tree.