Enable functions with multiple outputs

[henrus]

The low-level data-structures for this are in place.

However, there are currently some short-cuts in the the code which need to be ironed out.

BackwardMappingModel.error (TODO in line 636)
NonLinFitWithErrorContol.newPointsFWAction

In particular, the parameter fitting needs to have a loop over outputs, ie. do a fit per output.