BUG: Stop returning error when all vectors are zero for Siemens
Merge pull request #51 from Chaircrusher/DWIConvert_Fix

BUG: Stop returning error when all vectors are zero for Siemens
ENH: add short flags for inputDicomDirectory & outputVolume
BUG: Handle abnormal case of Siemens missing gradients

If the input DICOM series is completely without gradients/b values,
zero gradient vectors were added twice, so there were twice as many
gradients in the headers as volumes in the file.