refs #1700 time tracking

 * bug fix: min/max generation time didn't work in nested data tables because the column aggregation operations were not passed to sub tables
 * ui improvement: very small values were shown as 0s. use three decimal places if value is smaller than 10ms (e.g. display 2ms as 0.002s instead of 0s)

core/DataTable.php

@@ -449,7 +449,9 @@ class Piwik_DataTable
                 // if the row has the subtable already
                 // then we have to recursively sum the subtables
                 if (($idSubTable = $row->getIdSubDataTable()) !== null) {
-                    $rowFound->sumSubtable(Piwik_DataTable_Manager::getInstance()->getTable($idSubTable));
+                    $subTable = Piwik_DataTable_Manager::getInstance()->getTable($idSubTable);
+                    $subTable->setColumnAggregationOperations($this->columnAggregationOperations);
+                    $rowFound->sumSubtable($subTable);
                 }
             }
         }

core/DataTable/Row.php

@@ -282,6 +282,7 @@ class Piwik_DataTable_Row
             $thisSubTable = new Piwik_DataTable();
             $this->addSubtable($thisSubTable);
         }
+        $thisSubTable->setColumnAggregationOperations($subTable->getColumnAggregationOperations());
         $thisSubTable->addDataTable($subTable);
     }
 

core/Piwik.php

@@ -1437,7 +1437,8 @@ class Piwik
         $minutes = floor($minusDaysAndHours / 60);
 
         $seconds = $minusDaysAndHours - $minutes * 60;
-		$seconds = round($seconds, 2);
+        $precision = ($seconds > 0 && $seconds < 0.01 ? 3 : 2);
+		$seconds = round($seconds, $precision);
 
         if ($years > 0) {
             $return = sprintf(Piwik_Translate('General_YearsDays'), $years, $days);

tests/PHPUnit/Core/PiwikTest.php

@@ -91,6 +91,7 @@ class PiwikTest extends DatabaseTestCase
             array(1.342, array('1.34s', '00:00:01.34')),
             array(.342, array('0.34s', '00:00:00.34')),
 			array(.02, array('0.02s', '00:00:00.02')),
+            array(.002, array('0.002s', '00:00:00')),
 			array(1.002, array('1s', '00:00:01')),
 			array(1.02, array('1.02s', '00:00:01.02')),
 			array(1.2, array('1.2s', '00:00:01.20')),