Improve TF1::Save and TF1::GetSave in case the function has a parent histogram.

The value of teh function is saved at the center of the histogram bins instead of equidistant points. This makes drastic improvements when rendering the function in case of variable
bin size histograms.


git-svn-id: http://root.cern.ch/svn/root/trunk@31184 27541ba8-7e3a-0410-8455-c3a389f83636