Draft: Rename Neptun code to general identifier

Open Máté Cserép requested to merge 72-rename-neptun into develop

Started working on #72. Work in progress, but mostly done. Feedback could be made meanwhile.

  • Rename DB column
  • Modify DB column type
  • Rename fields and variables
  • Update unit and API tests
  • Update messages
  • Update comments and documentation

Closes #72

Edited by Máté Cserép