Implement text clipping in TASImage as requested [here](https://sft.its.cern.ch/jira/browse/ROOT-4538).
  Also the text size in batch mode for png (gif jpeg) files better matches the
  size on screen and pdf.

Add corresponding test

Fix TH1::ExtendAxis for the multi-dimensional case. The underflow/overflow should not be dropped if they belong to the axis which is not extended.
Improve stress histogram by adding new tests for merging with extendable axis

Set automatically Sumw2() when scaling an histogram. Add option "nosw2" if this behaviour is not wanted.

Fix a bug in the definition of the range for  a coefficient a1 in the Roofit tutorial and stressRooFit.
The parameter was considered constant instead should have been varying.
Need to update stressRooFit file since the fit now are different (parameter a1 is varying in the fit instead of being constant )

by Olivier

The support for merging labels for multidimensional histogram is added and is via TH1::Merge, so the TH2::Merge and TH3::Merge functions are now removed.
Fix also a bug in LabelsInflate
Improve stressHistogram tests
This should fix ROOT-8320 and ROOT-7320

Move the treatment of "-h" option before the call to TApplication in order to get the stressGraphics help when it is invoke with "-h".
Otherwise it displays the help of ROOT itself.