Fix/statistics

It seems really strange that Histogram has getMean/Median/Mode and then exposes generateStatistics.

However, generateStatistics() needs two passes, as it needs to know the mean before it can calculate the standard deviation. So it's less than idea to iterate over the raster twice in those cases where SD is not required.

It's possible to rewrite it into a class that would hang on to Histogram (either as a reference or as inner class) and perform lazy calculations, basically generating min/mode/max/min as the first pass and SD as the second when/if required.

Not clear if that amount of complexity is justified at this point though.