Allow updatable field to be False

[victorventuri]

I liked the idea of updatable being either False (to indicate a fully frozen HealthVariable) or a tuple of strs containing fields that can be modified. While we can use an empty tuple to indicate nothing can be updated, I feel like it's more intuitive to get a False when looking at that attribute.

[victorventuri]

Also, updatable needs to ordered, and not just a set, to assure all updates to internal variables are performed in the same order.

[WardLT]

I think this existing implementation already provides these abilities:

1. Clearing the set (updatable.clear()) is equivalent to setting it equal to False
2. Order is assured because it uses the order of fields in the class when outputting rather than the order in updatable

Is that sufficient?

[victorventuri]

Yep, thanks!