Move all macros (defines) to the .cpp module and make readChannel private

The header file is now as clean as can be. This is the class interface.
Nothing more must be accessible for the outside world.
The channel values are (16 bit) signed
Split readChannel into a function that reads raw and one that does voltage
Removed hardware folder, will be added later to a dedicated hardware repository.
Merge remote-tracking branch 'keesb/for-nick'