SAS Specification Class
A SAS specification is an object that can be queried to get the sas function at each timestep. This allows us to specify a time-varying sas function in a couple of different ways.
The Fixed spec is the simplest, producing a time-invariant function
The StateSwitch spec switches between several sas functions, depending on the state
of the system at a given timestep
The Weighted spec produces a weighted average of several sas functions. The weights can
vary in time