Allow multiplots in matplotlib

[fdarricau]

Adding tools to be able to do cleaner plots using matplotlib tools, plus a test to show it.

In detail, this commit allow users to:

• print well-defined titles for their plots (beware of calling command after the actual plot, it is a matplotlib mandatory and it won't work otherwise)
• print several curves on the same figure in a multiplot style, instead of merging all into a single plot, which leads to poor readability

[bchretien]

This should follow the naming conventions of RobOptim: isPlot_.

[bchretien]

I think a std::pair<unsigned int, unsigned int> would be better to specify the layout, rather than forcing the plot on a single column.

[bchretien]

👍