Avoid calculations unless we are really checkpointing the model

https://github.com/MetOffice/lfric_apps/blob/ff1ce36d2ef98d5e3da3acbac9a7f70677ca115f/science/gungho/source/driver/gungho_init_fields_mod.X90#L1105

The code in the if (checkpoint_write) block is called every timestep resulting in unncessary calculation.

Probably, what needs to be done is this:

1. The creation of the checkpoint times list needs to be moved to somewhere in initialisation.
2. The if-block needs an extra case to check on whether this is actually a checkpoint time. There is a `checkpoint_time` function in `lfric_xios_write_mod.F90` in lfric_core (which is currently private, so a linked ticket will be required).