Converts string constants representing trust direction into integers.

trusts edgeworth_trusts or trapezoid_trusts hyperparameter of tfl.layers.Lattice layer.

A list of trust constraint tuples of the form (feature_a, feature_b, direction) where direction can be -1 or 1, or the value None if trusts is None.

ValueError If one of trust constraints does not have 3 elements.
ValueError If one of trust constraints' direction is not in the set {-1, 1, 'negative', 'positive'}.