Allow same GPIO for switching and sensing

[xaver-k]

What does this PR do and why is it necessary?

The changes allow to re-use the switching GPIO for sensing the current on/off state. This addresses #182.

How was it tested? How can it be tested by the reviewer?

Tested on my personal setup, which uses a Raspberry 4 and pulls GPIO 2 to GND for switching the printer on:

Further testing on other setups is needed, but I only have the one printer.

What are the relevant tickets if any?

182

Further notes

• The changes also clean up some inconsistencies in the code between the sensing and switching pin (both use CdevGPIO now).
• The changes use the inverted-parameter of CdevGPIO to remove a lot of manual handling of inverted pin logic.
• See this PR as a starting point. I would highly suggest to do some refactoring and splitting up the logic for each switching and sensing method. However, I did not want to put a lot of energy into that without knowing whether you would actually be interested.

[kantlivelong]

RE: Refactoring, could you provide more detail in a separate ticket? Code definitely needs some general refactoring(format, make more use of constants, simplification, etc..) but not sure where your going with splitting things.