Loading: Simplify 'multipath' option for decadal loading module

Right now, the decadal loading module uses a complex work-around in order to be able to load dcppA-hindcast and dcppB-forecast years together in the same array, and then adjust the metadata accordingly.

I have found a good Start() call that should work for this case (and possibly all cases?), see: https://earth.bsc.es/gitlab/es/startR/-/merge_requests/236 and https://earth.bsc.es/gitlab/es/startR/-/issues/202#note_294583

The next step would be trying to apply this to the decadal loading module and getting rid of the unnecessary workarounds. This would also simplify the usage of Compute().