Adjust to changes in ProofAttemptConfiguration

[eugenk]

There is now a one-to-one relation between ProofAttemptConfiguration and ProofAttempt and between ProofAttemptConfiguration and Theorem (goal). This changes the JSON output:

On the serialization of ProofAttemptConfiguration:

• Remove number (it is now the same as the ProofAttempt number)
• Replace selected_theorems (reference to an Array) by selected_theorem (reference to a single Theorem)