Expanded output of task processing

* partly copy "task" nodes
** augment with result of performing the task
* executions of single transformations which expand into multiple target files are now grouped into tasks and subtasks
** a total result value is determined
* this change was implemented to aid the implementation of a "generation-digest" view
** such a "view" summarizes the results of task processing and may be configured in e.g. verbosity and error output

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