* Rearragement of TCandle-code - split into calculate and paint
   * Implementation for a "raw-data candle" inside TCandle - to be used from TTreeViewer in the future
   * Implementation of 1D histograms along each candle (left, right and violin) - to be used for violin-charts
   * Implementation of a zero indicator line for TCandle - to be used for violin-charts
   * Reimplementation if THistPainter draw option VIOLIN
   * Implementations of presets and individual options for VIOLIN-charts
   * Implementation of VIOLIN-charts in THStack - can be combined with CANDLE
   * Update of the docs (THistPainter and THStack)
   * New tutorials

Signed-off-by: Bertrand Bellenot <bertrand.bellenot@cern.ch>

More work was needed to:
- Create the index.md file
- Make the source files are Doxygen Compliant (formatting, Math formulae)
- Spell check

  histograms or graphs are painted in the same canvas thanks to the option "SAME"
  via a `THStack` or `TMultigraph` it might be useful to have an easy and automatic
  way to choose their color. The simplest way is to pick colors in the current active color
  palette. Palette coloring for histogram is activated thanks to the options `PFC`
  (Palette Fill Color), `PLC` (Palette Line Color) and `AMC` (Palette Marker Color).
  When one of these options is given to `TH1::Draw` the histogram get its color
  from the current color palette defined by `gStyle->SetPalette(…)`. The color
  is determined according to the number of objects having palette coloring in
  the current pad.

(cherry picked from 85f38d54)

- TSpectrum.cxx now refers to the new examples.
- Remove useless pictures.

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 )