Discrepancy between minROPP and LevMarq minimisers

[Ian Culverwell]

ROPP5 testing has shown large differences in retrievals using these two minimisers (eg see #232).

Huw Lewis documented similar differences (if not quite as bad) in GSR 6.

We should find out why minROPP and LevMarq differ.

Here are some preliminary thoughts:

Plots of the convergence of both schemes (in terms of J and dx, the normalised change in the state from one iteration to the next) are listed below, for the 8 profiles in IT-1DVAR-03, for reference with Huw's earlier work in GSR-03 and GSR-06

Refrac, J

Refrac, |dx|

Bangle, J

Bangle, |dx|