Add return_node_posteriors argument.

[hyanwong]

And deprecate the return_posteriors argument.

The return_node_posteriors should return a dict, each value of which is a numpy array of length num_nodes, and

1. in the case of discrete-time methods is keyed by timepoint,
2. in the case of the variational_gamma method is keyed by "shape" and "rate"

Note that 1. is transposed compared to the current return_posteriors argument. See #388