Week chunking

DateRangeChunker for prepare-once reports.

$chunker = new \ReportKit\Core\Date\DateRangeChunker();
$chunker->validateDateRange($start, $end, 6); // null|error
$weeks = $chunker->getWeeklyRanges($start, $end);

Downloads should compose from prepared store data and must not re-query the full range.