Set the choice for the state variables of the ion channel around which to linearize.
Note that when adding an ion channel to the node, the default expansion point setting is to linearize around the asymptotic values for the state variables at the equilibrium potential store in self.e_eq. Hence, this function only needs to be called to change that setting.
channel_name (string) – the name of the ion channel
statevar (dict) – The expansion points for each of the ion channel state variables