Add conductive flux limiting for vertical thermodynamics when calc_Tsfc=.false. #556
Description
When the vertical thermodynamic solver is forced by prescribed surface fluxes (calc_Tsfc = .false.), it can become numerically unstable. This instability appears to arise in the vertical thermodynamic solve when the conductive flux is too large (which depends on ice thickness and time step). Flux limiting is needed to stabilise the solution in this configuration.
An existing implementation of this flux limiting is available in an ACCESS fork of CICE5. Incorporating a similar approach here would enable stable coupling with the UM.
The conductive flux limiting occurs both before the thermodynamic solve if the conductive flux to ice thickness ratio exceeds a certain value, and during the thermodynamic solve if any of the layer temperatures exceed 0C.
See the implementation in this draft PR. Note these changes don't have any effect when calc_Tsfc=.true. and therefore pass the icepack tests.