Optimise plot size

[pelson]

Because cartopy uses a fixed aspect ratio in mpl, the actual size of the axes in pixels depends on the viewLim values. Instead, make it possible to define the ideal size of the plot, constrained by just one of the bounds in viewLim.

This will involve close mpl work.

[pelson]

A possible solution: http://stackoverflow.com/questions/15480113/force-aspect-ratio-for-a-map

[pelson]

Looks like the SO gives a pretty good answer to this. Closing until further requirements are available.