No error is thrown when perturbation diagnostic is used without based state

[tommbendall]

The Perturbation diagnostic takes the difference between a field and some base state (which has the suffix _bar).

We therefore need to set up the reference _bar field, which is done automatically for prognostics. However it needs manually adding when we take the perturbation of a diagnostic. However no error seems to be thrown if the user forgets to manually add the reference field!