can't use the add_to or use_abs options with color images.

Thanks to Volker Härtel for finding this.

optimizations are enabled "warning: assuming signed overflow does not occur
when assuming that (X + c) < X is always false"

rather than converting to HSI and filtering just the I channel.  Also, the
spatially_filter_image_separable_down() routine now only allows grayscale
output images.

when used on Windows.

one_vs_all_decision_function.

multiclass_linear_decision_function.

error message if they mistakenly use matrices that are not column vectors with
the general purpose optimizers.