Small improvements to Data Acquisition, Scheduling and GPIO modules.

Data Acquisition module:
- Get rid of static qualifiers that add complexity to code and are useless if user use the module correctly (i.e. does not instantiate its own object).
- Add guards to prevent accessing module functions when it is not started.
- Rewrite functions comments to ease API understanding.
- Corrected voltage offsets that were invalidated by conversion formula change.

Scheduling module:
- Add an initial value of "uninitialized" for interrupt source.
- Use typedef enum instead of enum class: prefix not mandatory any more.

GPIO module:
- Just add small comments to public include file to better understand file sections.