new methods set_xlabel and set_ylabel in "class Figure"

Very crude attempt to implement a figure-wide x and y label. I think that this can be useful for figures with multiple plots with same x and y axis.

As now the methods check for the absolute extrema of all the axes/subplots in the figure, and write the label as a text: xlabel below the lower axes and ylabel in vertical on the left.
In both cases the labels are centred with respect to the total area occupied by the axes/subplots

The current implementation has some (important) shortcoming:
1) the methods must be called at the end
2) they do not interact with tight_layout 
3) the offset from the axis must be set by hand and change according to the font size and the number of digits on the y axis

A way to solve this problem would be to create an empty axes enclosing all the other axes/subplots and to show only the x and y labels.
If anyone has pointers/suggestion on how to make those two methods more in the line of matplotlib and useful to the general user, please let me know. I'm happy to work on them