converter usage

 usage :
 (1)   ./converter [option] mat
 (2)   ./converter [option] -

 options can be:
   -n : convert from new format to old format. [default: old to new]
   -s : sorts the row by weight (smallest first) [default : enabled, new format only]
   -S : don't sort the rows by weight (smallest first)
   -r : reverts order [default : enabled, new format only]
   -R : don't revert order
 (1) will produce a file on stdout from matrix mat
 (2) will produce a file on stdout from  stdin
 this would happen like : zcat mat.gz | ./converter mat - > mat.gbm
 Warning : the input matrix has to be in the correct format
 Warning : new matrix format is suffixed by .gbm (but not necessary)