Scenes without main relation

[danielhers]

TUPA often predicts a node with A (participant) children but no P/S (process/state). There are several ways to tackle this problem:

1. Add constraints on Reduce to force creating this relation. Might make the parser get to dead ends with no possible action, though, or resort to taking unreasonable decisions. Beam search might remedy this (Implement beam search #10).
2. Attach the missing relations in post-processing.

[macleginn]

As simpler captions show, sometimes it is actually necessary to have a relation-less scene ("A man on a beach"), unless we want to obligatorily have P/S. The latter is a valid strategy, IMO, but it will entail creating implicit P/S nodes, which are not used at the moment.