Add support for the old Olympus packed/unpacked formats

Added rawspeed support for all the Olympus cameras at rawsamples.ch. This ended up including several cases:

* A packed 12 bit format with a control byte every 10 pixels (similar to the Panasonic one so code for that was refactored out of Rw2Decoder into RawDecoder)
* Unpacked big endian and little endian formats. The big endian one was left aligned so a new decoding function was needed
* A packed and "interlaced" format where odd/even lines are encoded in two consecutive sub-images

I've tested this in the context of darktable and it now supports all the ORF files at rawsamples.ch