#43 omlib: use ComputeStatistics for min max in gdal raster

[Tim Sutton]

When computing min max for a layer, we should not be using GetMinimum, GetMaximum calls in GDAL as they are not guaranteed to return the definitive result (they may be estimated in this case). Rather we should use ComputeStatistics:

http://www.remotesensing.org/gdal/classGDALRasterBand.html#131debfcbb9b0f87bd1a019790192802