backend hanwatts prediction model web interface

Merge pull request #27 in UPRFIR/backend-hanwatts-prediction-model-web-interface from feature/UPRFIR-201-alleen-nieuwe-data-ophalen-ipv-alles to develop

* commit 'b069097f22154f8649710f0478442e96eefc2693':
  Add check if no already exists
  Add fetch all existing ids