basecurve tool: fix PPM header reading code for PPMs generated by recent imagemagick
basecurve tool: optimize a bit making the 8bit/16bit to float only once
basecurve tool: rename some vars
basecurve tool: tentatively support orientation in helper script
basecurve tool: update README
basecurve tool: massive refactor of the helper script

- refactored the helper script. It is now also usefull for non
  Nikon/Canon raw files as long as it given the JPEG file as
  argument too.
- added support for the -s option in the helper script as well
- updated accordingly the README file
Merge pull request #562 from edgomez/dt_curve_tool_fix

basecurve tool: fixes and improvements