DNG BlackLevel handling when cpp != 1

@LebedevRI @alexvanderberkel I got darktable decoding dual pixel dng files.

There is one more thing in rawspeed that "requires relaxing". bool DngDecoder::decodeBlackLevels(const TiffIFD* raw) { in DngDecoder.cpp should accept cpp==2 too.

So i suggest it to be

 if ((mRaw->getCpp() < 1) || (mRaw->getCpp() > 2))
    return false;

Can you agree here? And could you modify this or do you want me to do a pr?
BTW it would be nice to push current rawspeed to dt :-)

Originally posted by @jenshannoschwalm in https://github.com/darktable-org/rawspeed/issues/214#issuecomment-570887227

More specifically, as per https://www.adobe.com/content/dam/acom/en/products/photoshop/pdfs/dng_spec_1.4.0.0.pdf, all that blacklevel info is not at all specific to cpp=1/cpp=2.
See BLACKLEVELREPEATDIM, BLACKLEVEL, BLACKLEVELDELTAV, BLACKLEVELDELTAH.
Which only highlights how broken the current 4-channel mRaw->blackLevelSeparate is.

Start in DngDecoder.cpp at DngDecoder::decodeBlackLevels and read the DNG 1.4 specification sections for BLACKLEVELREPEATDIM, BLACKLEVEL, BLACKLEVELDELTAV, and BLACKLEVELDELTAH. Trace how mRaw->getCpp() and mRaw->blackLevelSeparate are used, then verify that black-level metadata is handled for cpp values beyond the current restriction without breaking existing decoding behavior.