Query: Added ~IfValid methods to Element types. This allows creation of filters that can operate on vectors and scalars without nodata values infecting the data; this is especially useful for reductions and kernels, as it prevents dilation and erosion of nodata. Updated Blur and Variance filters to use this technique.
Query: Added method to update the validity of an Element, but only if the value being set is valid. Allows a filter to start with an invalid element and let it become valid later if a valid pixel is found. Applied this to the Mean filter.
Merge remote-tracking branch 'origin/master'