Merge branch 'headers' into 'master'

MessageParser overhaul

This MR:
*  Changes the name of some `MessageParser` functions
*  Implements most of the **CCSDS space packet** and **ECSS TC & TM** headers, along with their tests
*  Adds documentation for all ECSS Definition constants
*  Removes template from all `Message::appendString()` functions
*  Add a `Message::appendMessage()` function

See merge request acubesat/obc/ecss-services!46