Subfields not included in diagnostic field dependencies

[Adam Candy]

Subfields, the fields representing terms in prognostic equations, such as absorption in a tracer equation, are not included in the diagnostic dependency ordering.

An illustrative example:
Prognostic scalar equation 'Salinity', with an absorption subfield 'SalinityAbsorption'.
The absorption field in the equation for salinity is required in the calculation of a diagnostic field, but the code errors in the dependency considerations. The field 'SalinityAbsorption' is not known as available.

I think the issue is in the preprocess checking only, and the calculation of the diagnostic would go ahead fine (to be confirmed!). Tricky part is to integrate adding these with the dependency ordering correctly.

Standard fields are accessible, and the subfields should be included here too.