WIP: A vtkDICOMWriter class for writing DICOM images.

This will be merged when it is capable of writing these SOP classes:
1.2.840.10008.5.1.4.1.1.7    Secondary Capture
1.2.840.10008.5.1.4.1.1.7.2  Multiframe Grayscale Byte Secondary Capture
1.2.840.10008.5.1.4.1.1.7.3  Multiframe Grayscale Word Secondary Capture
1.2.840.10008.5.1.4.1.1.7.4  Multiframe True Color Secondary Capture

After that, the next step will be writing modality-specific SOP classes,
though validation of those will likely be the responsibility of the
application, rather than the vtkDICOMWriter.