Add protected default constructor for CallbackStore.

[brettle]

This is to allow 3rd party sim providers (eg vendors) to subclass this class so that the register methods of their sim classes can return CallbackStores like the builtin sims.

Although it was already possible to create such a subclass but passing dummy parameters to one of the other constructors, this eliminates the need to pass such dummy parameters and makes it clearer that subclassing is allowed.

[PeterJohnson]

/format

[PeterJohnson]

Needs a formatting fix.