add DICOM (aka supp 145) support #142

[Benjamin Gilbert]

On 03/23/2015 10:28 AM, Mathieu Malaterre wrote:
> 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.

Once a quickhash is defined for a given slide file, it can't be changed. 
  As a result, it's better for the very first quickhash implementation 
to support the full generality of the file format.  Otherwise, a later 
version of your driver might have to select between two different 
implementations depending on the properties of the slide file, which 
complicates the code.

For example: the Leica driver has to use one quickhash implementation 
for slides with a single AOI, and a more general implementation for 
slides with multiple AOIs.  We couldn't use the general version 
everywhere because that would change the hashes of single-AOI slides.

--Benjamin Gilbert