Add a function for explaining forecast models, explain_forecast.

[martinju]

TODOs:

• [x] Document explain_forecast
• [x] Update vignette with usage example for explain_forecast()
• [x] Consider modifying setup to work with forecast instead of creating a very duplicate function (MJ will look at this)
• [x] Handle failing checks (MJ)
• [x] Go through and approve changes in other functions (MJ)
• [x] Handle weird conflict between imported functions from quantmod/zoo packages
• [x] Handle issue different arguments in generic function predict_model
• [x] Consider moving extra and output_size to internal
• [x] Do we need both extra$horizon and output_size?
• [ ] In the end: Consider making some argument names more informative (e.g. type -> explain_type, horizon -> pred_horizon)

[martinju]

After merging another branch into devel I did some updating to make tests and checks pass again. Nothing major related to what we do in this PR.