Consider adding 'global' variables to curriculum.

The basic idea would be to add an optional data class that could track variables common to all, or multiple, stages. For instance certain animals may be trained in a mirror contingency to counter balance for motor output. This would be used in all tasks but would remain constant without animal, so the default should be constructed from this value.

It should be noted that this can already be implemented by the user by constructing a different stage per animal. This is somewhat a bit clunky however.