Moved processed metrics computation to DataTableGenericFilter, removed new filters & implemented as methods in Report to avoid confusion regarding re-use, allow adding processed metrics as DataTable metadata and use this to rewrite AddProcessedMetrics filter, correct name of Metrics::getMappingFromIdToName function, added placeholder AggregatedMetric class for future, revise Metric/ProcessedMetric hierarchy (add methods for translation/etc.), allow .get API methods to use metadata to automatically figure out which columns to select, get EcommerceOrderWithItemsTest to pass.

Add comment to screenshot testing for when no tests can be found telling user about --plugin parameter.

Add more missing metrics from VisitFrequency.get API output (including max_actions_returning, nb_visits_converted_returning, etc.).

Add missing nb_users_returning metric to VisitFrequency.get Report metadata class so it will appear in 'processed' reports.

refs #6511 fixed some tests which shows it seems to work now. I presume the expected results are correct but I do not know. Also updated another test where someone committed his path from his local server