The simulated momentum is different from the expected momentum (n-layer physics)

[edoddridge]

As indicated in #99, the tests developed for #66 have identified a discrepancy between the expected momentum and the simulated momentum in the n-layer configuration that is approximately a factor of four. This, presumably, includes the error described in #99 as well as something else within the barotropic correction step.

[axch]

Same labeling rationale as https://github.com/edoddridge/aronnax/issues/99#issuecomment-295060593, except with a higher degree of belief that an actual bug is involved.