ENH: Update to ITK version of DCMTK.
ENH: Update ITK to version that contains internal version of DoubleConversion.
ENH: DoubleConvert funtionality from ITKv4

After working with Bill Lorensen, the DoubleConvert
features are added to ITKv4, so using that version of
DoubleConvert in order to minimize the dependancy chain
necessary to provide that functionality.