[HACKTOBERFEST] Update to pi4j-v2 & Multi-Chip support

[MMMMMNG]

Changes

• Update everything to pi4j-v2
• Add multipleNewOnSameBus() factory method. This means multiple chips can now be used on the same bus, using hardware address pins.
• Add automated testing with an updated version of the pi4j-mock-spi-plugin (there was a bug in the transfer() method, see #301. Since those changes aren't in maven central yet I bodged together a workaround solution with github actions, see the [...]-SNAPSHOT branch)

NOTE:

The Interface for the static factory methods changed:

//old
public static MCP23S17 newWithInterrupts(SpiChannel spiChannel,
                                             GpioPinDigitalOutput chipSelect,
                                             GpioPinDigitalInput portAInterrupt,
                                             GpioPinDigitalInput portBInterrupt) 
//new: ChipSelect and Channel are all handled by the single Pi4j-v2 class `Spi`
public static MCP23S17 newWithInterrupts(Spi bus,
                                             DigitalInput portAInterrupt,
                                             DigitalInput portBInterrupt)

Why

We used this library (or my updated version of it) extensively in our project, so by the end I was quite familiar with it. When I heard of Hacktoberfest, I thought this was the perfect opportunity to contribute what I can offer.
@Ro5bert I would appreciate it if you could label this PR "hacktoberfest-accepted" (if you like it, that is), that way it counts towards my hacktoberfest contributions.

Taken from https://hacktoberfest.com/participation/

Thanks!

Feel free to provide feedback and I'll try to get to it asap.

Have a good one!
Cheers!

-MNG