WIP on isotonic_calibration PR

Hi Alexandre,
this is the discussed PR which
- renames calibration_plot to calibration_curve
- fixes a supposed bug in plot_calibration_plot.py (please check, the calibration curve is plotted transposed)
- adds an histogram to plot_calibration_plot.py
- add normalize option to calibration_curve
- does some polishing of the plot_calibration.py example
Let me know if there are further issues in the isotonic-calibration PR where I can support you