Fix three vtkDICOMCompiler errors for writing metadata.
Fix three vtkDICOMCompiler errors for writing metadata.

Elements of type AT were written incorrectly due to a math error.
PixelData size was computed wrong if SamplesPerPixel was absent.
Computation of sequence item size missed the 8 bytes for item tag.

hendradarwin/vtk-dicom,hendradarwin/vtk-dicom,hendradarwin/vtk-dicom,dgobbi/vtk-dicom,dgobbi/vtk-dicom,dgobbi/vtk-dicom