Get rid of LigandOnlyModel?

[kaminow]

The way we have the LigandOnlyModel/GAT model set up is kind of a weird special case of the regular Model. We could probably rework the GAT._get_representation and GAT._get_energy_func methods to work the same way as they do in the structure-based models, and then have the strategy be a ComplexOnlyStrategy, so we only process the one representation. Either way, we should probably move the self.readout call into the representation, since the current representation for GAT outputs an mxn array, where n is the feature dimensionality and m is the number of nodes. This is not the way we have things setup for everything else, where the output of the representation should just be an n-dim vector.