correlation() methods to output the wrong answer if the covariance was
negative.

the order of function argument evaluation is not defined, the test had slightly
different results on different systems which could cause the tests to falsely
indicate failure.

solver.  This makes some things, such as the structural_object_detection_trainer,
significantly faster.

non-negative weight learning support.

suggested by Platt's paper.

sparse vectors to avoid unnecessary RAM usage in this case.