Additional methods for switching to height-varying gravity

### Are there any linked Issues or Pull Requests?

https://github.com/mo-cjsmith/lfric_apps/tree/gravity_switch

### Brief description

Although the shallow atmosphere approximation is not used by the UM(ENDgame) , it still retains the constant gravity field of earlier models. If LFRic is to run with true height-varying gravity, initialisation from a UM start-dump must take account of the change in gravity between the two models to avoid a significant change to the meteorology of the data.

The existing code to do this has a known defect, which is to be corrected here. An additional variation of this method is added, along with the option to use a method that takes another approach to the problem.

### Further details of the issue.

_No response_