Add support for forecast forcings for LAM setup

[inakbk]

TODO:

• set up test case
• make bris require all forcings defined in the checkpoint file (unless overwritten)
  • endre loopen i advance_input_predict
• I config: nytt element som heter dataset-forcings e.l.
• I DataModule eller separat DataModule: laste inn dataset-forcings
• I advance_input_predict: Lese forcings fra dataset-forcings