Add a flag to model.save_to_h5 to prevent overwriting

[allevitan]

Sometimes by accident we've found issues with folks overwriting files from previously saved reconstructions during beamtimes when scripts are shared.

I think it would be nice to change the default behavior of model.save_to_h5 to avoid overwriting past work.

I propose adding a safety_mode=("overwrite"/"increment"/"prompt") flag to the function, which would by default be "increment".

False or "False", just overwrite, but print a warning
True or "True", add an increment to the saved filename
"prompt" - ask for user input as to which behavior to use!