Corner case bug spotted (thanks Luca Jovine)

If you

• use xinfo files
• use -2d
• do not set START_END

you may find you get a weird complaint about detector_class.

This is easily resolved for the moment by setting START_END to correspond to the first and last image you want to process... I need to fix this ;o) it did not used to work like this...

Thanks to Luca for pointing this out and patience in debugging...