vector_to_matrix(), pointer_to_column_vector(), and pointer_to_matrix() to
mat() (in addition to adding a few more mat() overloads for certain things).

pointer_to_column_vector(), and pointer_to_matrix() methods all into one
convenient interface.  Also made stddev() slightly more general.

This change also deprecates the previous matrix conversion functions.

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.