Add support for per slice rescale slope and intercept

[ivmartel]

DICOM data can have a per slice rescale slope and intercept as shown in #79.

[howardm]

Thanks,

I've been trying to learn about the rescale intercept.

Here are six anonymized cases from my Teaching File from different institutions/devices:

https://www.dropbox.com/sh/zwulb580h3yci83/iijG45aIlv

I see that for some of them the value seems to change from -1024 to a value about twice that, and sporadically/randomly within a particular Series.

So this appears to be common.

I hope this helps...

Howard

[ivmartel]

Fixed with the merge of branch 80-slicersi (see #128).