Clarify SQL join column in queries
Clarify SQL join column in queries

In particular, with SQLAlchemy 1.2.14, the query
`session.query(NCExperiment.experiment, ...).join(NCExperiment)`
fails because it cannot figure how to construct the join. We should be
explicit about the column over which we're joining, to be sure they're
executed correctly.
OceansAus/cosima-cookbook