Users can't override default values for a signal's parameters until after it has been created

[colinbdclark]

Three Signals currently support "parameters" (i.e. modifiable but not non-time-varying values): Value, Accumulate, and Dust.

At the moment, parameters can't by provided by a user at initialization or construction time because they are automatically hard-defaulted within the signal's ```init()`` function. It's only possible to override a parameter's default value afterwards.