Make sure that all modules are consistent with respect to Bid initial conditions

[johnbachman]

Some of the Shen models don't have Bid initial conditions because the precise initial condition varies depending on the figure/simulation in the paper. This leads to confusion when composing the model with rec_to_bid and pore_to_parp, because the Bid initial condition must then be added.

Better to have a default initial condition for Bid that is consistent across models and that can be overridden.