2D: add 'aspect_ratio' attribute to deviate from fixed square scaling

The 2D learner treats x and y on equal footing now.

An adaptive user sent me this:

and a zoom in on the "interesting" features:

where there is more detail in y than in x.

This merge request allows you to set a new attribute ratio.

Examples:

learner = adaptive.Learner2D(ring, bounds=[(-1, 1), (-1, 1)])
learner.aspect_ratio = 1/4

learner = adaptive.Learner2D(ring, bounds=[(-1, 1), (-1, 1)])
learner.aspect_ratio = 4