Bring file_safety_check tests earlier in initialisation

[David Dickinson]

The input file_safety_check is used to trigger a test that we are able to write restart and/or distfn output files prior to attempts to advance the GK system. This is primarily designed to catch cases where users forget to create the restart directory etc. Unfortunately this test only occurs when we initialise diagnostics, which happens after we have initialised the equations – which tends to be expensive for problems where we want to save the restart files. It would be helpful if we could bring this check much earlier in the initialisation cycle. It might also be helpful to check if we can write response files (if requested).

Ideally we would also check that we are able to read restart/response files if requested.