ak/bk stability check - Xi Chen tool wrapper.

[FlorianDeconinck]

One concern (not related to this commit) is that there is no foolproof way to automatically generate a good set of ak/bk levels, and when creating an entirely novel set of levels some degree of hand-tuning is necessary to avoid stability problems. Xi Chen had developed a tool which can help, and interpolating from predefined level sets is usually pretty good, but some user discretion is needed when making new level sets.

There is no a priori way to ensure stability but typically we can examine the differences in delp and delz in a variety of situations, which is what Xi's tool does. We could run tests to ensure that the differences do not exceed some tolerance. This might be a good way forward; furthermore smoothing can be applied to improve the smoothness and stability of the selected coordinate.

( Per discussion in #36 )