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 :class:`Fixed` spec is the simplest, producing a time-invariant function
The :class:`StateSwitch` spec switches between several sas functions, depending on the state of the system at a given timestep
The :class:`Weighted` spec produces a weighted average of several sas functions. The weights can vary in time
.. automodule:: mesas.sas.specs
:members: