with incompatible function signatures you got a really confusing error message
from the compiler.  This change makes it so you get a much more informative
error message.

svm_rank_trainer.

expecting something that looks like a multiclass decision function.

make print_options() print related options so that they are grouped together.

end so that the user doesn't have to print them.

widgets.

more general.  Previously they required the user to use the same type of image
for both input and output.  This has been relaxed so now different typed images
can be used.