jpeg: Extended jpeg support

[feliwir]

This builds on #352

This adds support for process 4 of the JPEG specification (12-bit support). The decoding routines are similar to the existing 8-bit decoding routines, however i made the bitdepth a generic parameter. In theory everything in mcu_ext.rs could be applied to the functions in mcu.rs and then we could remove the duplicate code

For testing i added some publicly available DICOM files, provided by the standard committe. These files are publicly available here: ftp://medical.nema.org/MEDICAL/Dicom/DataSets/WG04/
It contains both extended jpeg (JPLY) files and lossless jpeg (JPLSS)

To access the pixeldata from these files i added https://github.com/Enet4/dicom-rs as a dev dependency

• Profile if the generic variants of the scalar IDCT calculation are equally fast and replace the non-generic variants
• Stop generating PPM image files in the tests. How to validate correct decoding?

[feliwir]

I made the IDCT functions generic. I think for the other functions we could do the same, but i am not sure how much generic usage you're a fan of