Support for ignoring/discarding input keys #4
Description
In some situations the user may want to discard a certain key for some reason altogether in histogram computation. This may occur for example when doing multiple passes over the data or when passing over padding data (last bytes in a row-allocation of a image may be outside the image).
The trivial workaround solution for this at the moment is to have one extra bin as a trash-bin and just assign all entries to be discarded there, but it might be faster if the algorithm itself can see that this key/value can be ignored (especially with higher number of bins).
The best interface for this feature is not completely clear, so think it through first. One option would be simply to make a convention that bin-indices less than zero are ignored and this option is turned on with flag (in order to avoid unnecessary checks in normal cases).