Make *computed* functions consistent

[bundfussr]

Currently the behavior of derive_param_computed() differs from all other functions: if dataset and dataset_add are specified, both datasets are used as input.

To make it consistent dataset_add should be mandatory and only this dataset should be used to derive the new records.

For the use case that more than one source dataset is required like for some questionnaires parameters derive_param_computed_msrc() should be implemented.

derive_param_computed_msrc( 
  dataset = NULL, 
  by_vars, 
  parameters, # list of parameter() objects 
  source_datasets, 
  set_values_to, 
  constant_by_vars = NULL, 
  constant_parameters = NULL, 
  keep_nas = FALSE 
) 

parameter( 
  dataset_name, 
  id, 
  condition = NULL 
) 

[bundfussr]

@manciniedoardo , why was this closed?

Wouldn't this be a good change for admiral 2.0?

[manciniedoardo]

sorry, i thought this fell under the review we did prior to 1.1 - will reopen