ENH: Export to DICOM format

[hina-shah]

Creating this as a separate issue because it seems like this might take more time in early investigation.

• Exporting a Scalar volume directly into dicom format is not supported in Slicer.
• Tried going down the DICOM library path, but it's not clear if this is possible
• Looks like going through pydicom might work:
  -- When exporting a series of dicom files as a single dicom image we can read metadata from the first file, and get pixel spacing from the import dicom volume using this piece of code
  -- Need to investigate what is the correct dicom metadata for a volume image.

[hina-shah]

Should be able to export as a single DICOM imager as well.