add DICOM (aka supp 145) support #142

[Mathieu Malaterre]

Folks,

I prepared a quick and dirty patch to support DICOM (aka Supp 145) in
OpenSlide. Some datasets have recently been released by DICOM WG26 and
I could test my code against those:

https://github.com/malaterre/openslide/tree/dicom-wsi

Long story short, this is not clear where this standard is going, but
I assumed that when exporting as DICOM VL Whole Slide Microscopy Image
Storage instance(s) a DICOMDIR would be generated. So the
implementation will require a DICOMDIR instance to process the slides.

While DICOM Supp 145 do support multiple slides per Study, support for
those has not been added since OpenSlide does not provide an API for
dealing with those -yet-. This means I can safely use the Study
(alone) to have a unique quickash.

Enjoy,
-- 
Mathieu