fischertechnik I2C port expander

Changelog:
2019-03-27: Added files for etching the PCB in extended gerber, PDF, and SVG file formats in archive PCB.zip (cf. last two images)

This easy to build PCB with printed enclosure provides an additional 8 digital inputs (or 25mA outputs) and 8 outputs with 500mA each, controlled by I2C. It is based on the [MCP23017](https://www.microchip.com/wwwproducts/en/MCP23017) 16-Bit I2C I/O Expander, the outputs are driven by a ULN2803.

The enclosure has connectors for the [fischertechnik construction toy system](https://www.fischertechnik.de/en) and uses my printed flush sleeve design ("[Printbuchse](https://www.thingiverse.com/thing:3375338)") for the fischertechnik typical 2.6mm jacks. The outputs can drive ft-parts like lamps, magnets, valves, or (undirectional, mono-speed) motors. Combine two or more of the ULN2803 driven outputs, if you need more than 500mA.

* The 2x3 pin I2C connector is pin compatible with other ft I2C cables ([ftDuino](http://ftduino.de/), TX).
* Loo