Clean up DAO

[CoderTobi]

• Add JSDoc to the functions
• Maybe use objects instead of so many parametrs?
• Make date handling consistent (either use strings or DateTime objects throughout the DAO)
  Create uses DateTime objects, Update uses strings
  Strings is probably better because the validation middleware uses strings
• change var to let (see change var to let #76 )
• Maybe change it that the DAO ensures the result is an array instead of the service
• Maybe move extendPlantJSON to DAO and clean it up