Fix normalization bug.

There is a bug in normalization that was introduced with 672aec002f5d2e5b53f78e0a2e836e1da8996ec6.

Fixes #747
Fixed bug with GeoTools reading NoData values from GeoTiffs.

Reading NoData values from interger-based rasters with no associated NoData values was broken.

Fixes #768
Added .statistics() to RasterSource
fix bug in Histogram.getMean

also switched it to use delta-mean method to avoid possible overflows
Added unit test for Histogram.getMode
Moved version up to 0.9.1