Add new dispatch_setup(model, :setup, modeldata)

[sjdaines]

There are now three dispatch_setup calls required, with attribute_name

• :setup - set up Reaction internal state, non-state Variables,
• :norm_value - set state Variables to :norm_value attribute
• :initial_value - set state Variables to :norm_value attribute

This avoids previous ambiguity about where non-state setup should happen