Make pin numbering scheme obvious without the need to read the docs

[anohren]

In the example in the readme we have

let gpios = SwiftyGPIO.GPIOs(for:.RaspberryPi3)
var gp = gpios[.P2]!

This innocent snippet requires you to read the readme to understand it, since despite literally passing the words "Raspberry Pi" you don't get the pin numbers for the Pi, but for BCMxxxx.

My suggestion is to make the api more self documenting by returning the Pi pins when passed a Pi, and the Broadcom numbers when passed a BCM device.

This took me some time to figure out since I was so sure the code was telling me that I was getting the Pi numbering.