DICOM conformance

This patch introduces a more strict policy for DICOM conformance. As before, slicebox is only accepting the basic imaging modalities when importing from PACS, directories or files, and also accepting secondary capture files when adding single images using the API (at /api/images). In this release, we are also restricting the number associated transfer syntaxes, typically allowing the following four:

• Implicit VR little endian
• Explicit VR little endian
• Explicit VR big endian
• JPEG baseline process 1 (standard lossy JPEG)

Slicebox may allow further transfer syntaxes in the near future such as lossless JPEG, run-length encoded images and deflated datasets provided we find sample images and codec libraries with compatible licensing.