Add unit tests for anonymisation process

[milanmlft]

Fixes #132

• Refactor: move add_private_tag to core.dicom_tags
• Only load project config when it's needed
• Add note to refactor private tag addition in orthanc-raw
• Add DICOM image test fixtures to test anonymisation
• Add anonymisation unit tests

[milanmlft]

The whole private tag class is very odd to me, but that was already in there and everything else looks good. 🚀

What do you think is odd about it? I guess it's just meant as a consistent interface to add custom private tags to DICOM datasets.

[peshence]

Yeah I had some misunderstandings about private tags that @HChughtai helped me out with so it makes a bit more sense now. Thanks both!