ENH: Always return complete profiles from sequence form-based methods

[tturocy]

When computing using the sequence form, actions do not need to be specified more than one deviation off of the equilibrium path to confirm an equilibrium.

In such sets technically we have sets of Nash equilibria, as any behaviour in those parts of the game is consistent with equilibrium.

As we do not at the moment have a way to represent such sets, we will do the following:

1. All sequence form methods will return profiles set to the centroid more than one deviation off of the equilibrium path (using e.g.UndefinedToCentroid).
2. Documentation should be updated to clarify this (possibly with an example somewhere to make the convention concrete).

In future we should have some way of indicating sets of equilibria, but that is a nuanced topic.