Merge branch 'general-fixes' into 'headers'

Fixes on warnings & errors

This MR performs a minor tidying of the ECSS-services repo:
*  Most doxygen warnings are fixed
*  Tests are now expected to return no errors, unless explicitly specified (Closes #19)

Note: !46 has to be merged before this.

See merge request acubesat/obc/ecss-services!48