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.

refs #5940 put tests in correct folders, better testsuite names, some tests still fail and I cannot figure out why

refs #6162 when aggregating periods, do not sum 'Unique Users' metric as this is not a valid operation.

Refs #6109 New core metric: nb_users as the count of distinct user ids. Means "Active Users". Displayed in Visitors Overview, in metrics picker, in API response, in Metadata

Piwik is a free/libre analytics platform. Refs #4455 GNU Package requirement to use free/libre instead of open source