basecurve tool: a fair amount of new feats

- feat: added tonecurve support
- feat: added multi image analysis to mitigate poor dynamic curve coverage on single images
- feat: retrieve make and model  from exif metadata
- feat: added a script for using embedded JPEG from Nikon/Canon raw files and automate stats gathering
- bonus: code fixes, code cleanup, better error paths

NB: The tool is now called dt-curve-tool.