Provide Ellison scale option

[jessecusack]

The Ellison scale is defined as the root mean square density in an overturning patch divided by the density gradient,

Le = <rho'>^0.5 / (drho/dz).

If the density perturbation is relative to a minimum potential energy background, then the Ellison scale can be used to estimate the available potential energy,

APE = 0.5 N^2 Le^2.

N^2 is also calculated from the background. The true background should be found by sorting the 3D density field. Observationally, the background is approximated by sorting a single density profile.

The Ellison scale may be a better estimate of the Ozmidov scale in some cases.