Address thread safety of read-only operations

[gidden]

Per our discussion in #97, read-only operations (e.g., just reading scenario data from a database) is at the moment not thread safe. In essence, there is a check_out() occuring in order to check whether the current model schema is at the expected version. We should address this either by guaranteeing thread saftey of read-only ops or by offering a thread-safe version (through key-word argument or something else).