PERF: Move the remaining of BeforeThreadedGenerateData() to GenerateOutputInformation() where it belongs

PERF: Remove calls to Update() and replace with calls to UpdateOutputInformation(), which will compute the stats without processing the full image result in memory

PERF: Move statistics estimation code out of GenerateData() to allow for a call to UpdateOutputInformatio() that will compute the stats but not the result on the full image.