Description:
------------
From what I can find it's currently not possible to add or subtract microseconds 
using DateTime::format. Every other unit of time can be added or subtracted. 
Please consider adding:

decisecond
centisecond
millisecond
microsecond

Test script:
---------------
$time = new DateTime('01:01:01.000001');

$timeH = $time->modify('+1 hour');
echo $timeH->format('H:i:s.u') . '<br>'; // prints 02:01:01.000001

$timeM = $time->modify('+1 minute');
echo $timeM->format('H:i:s.u') . '<br>'; // prints 02:02:01.000001

$timeS = $time->modify('+1 second');
echo $timeS->format('H:i:s.u') . '<br>'; // prints 02:02:02.000001

$timeMS = $time->modify('+1 microsecond');
echo $timeMS->format('H:i:S:u') . '<br>'; // error