Various image conversion scripts

Intended as a combination of frontends to RawTherapee to support niche file formats and unsupported cameras prior to libraw integration into RT, and also in the case of libraw2dng as a base for more advanced preprocessing scripts (such as average stacking)

imagecodec2tif was specifically designed to address the following RawTherapee issues: Beep6581/RawTherapee#6612 Beep6581/RawTherapee#6606 Beep6581/RawTherapee#1895