Improved flexibility of setting aspect ratios

Matplotlib allows numerical aspect ratios to be set, as well as ‘equal’
or ‘auto’. Now, the toolbar option will toggle ‘equal’ and ‘auto’, but
it’s also possible to set the numerical value from the command line,
using, e.g., ‘plotview.aspect=3’. Unchecking the toolbar icon will
revert the setting to ‘auto’.