Provide a way to automatically fall back to mocks if a real device is not detected

[FDelporte]

As reported by @DieterHolz

Lots of our student teams combine the RPi with a GUI, for example, using JavaFX. For them, it is very convenient to start the app on the laptop without any Pi4J exceptions, to implement and test the GUI and application logic, without connections to sensors, etc.

Current solution:

pi4J = (BoardInfoHelper.runningOnRaspberryPi()) ?
        Pi4J.newAutoContext() :
        Pi4J.newContextBuilder()
                .add(new MockDigitalInputProviderImpl())
                .add(new MockDigitalOutputProviderImpl())
                .add(new MockPwmProviderImpl())
                .add(new MockSpiProviderImpl())
                .add(new MockI2CProviderImpl())
                .build();

It would be great if something like this is incorporated in newAutoContext or some similar method.

[stefanhaustein]

I don't think we'd want an AutoContext with mocks by default -- we'll want an error if the code won't work for real.

That said, how about adding "newAutoContextOrMock()" somwehere/somehow -- ideally in a way that doesn't make production code depend on test infrastructure by default? O:)

[DigitalSmile]

I would add to Stefan words - I really do not think we need any context at all.

[stefanhaustein]

This seems to be related to #616