automatically load band stats if they exist

[etiennesky]

This would be useful e.g. in the value tool when opening the graph.

In the gdal provider, load the cached stats (if exists) in the constructor. We could fetch the cached stats (bForce=FALSE) like this

CPLErr myerval =
GDALGetRasterStatistics( myGdalBand, bApproxOK, FALSE, &pdfMin, &pdfMax, &pdfMean, &pdfStdDev );

We could also add a hasCachedStats() function and/or add a bForce argument to bandStatistics

[blazek]

There are also QgsRasterDataProvider::minimum/maximumValue() and it is not well defined what they do. Currently those methods are used for load estimated min/max in raster properties, but GRASS provider for example, is giving precise min/max. OTOH, GRASS does not have all the statistics ready (mean, stdev) so it cannot say hasStatistics().

We could add MinMax to Capabilities.