def init_params() method for channels

[michaeldeistler]

Problem

In many cases, one would want parameters to depend on one another. For example, if one has

channel_params = {"temperature": 37, "gNa": 0.1}

then the sodium conductance would depend on temperature via:

effective_gNa = 2.3 ** ((temperature - 37) / 10) * gNa

Proposed implementation

Together with @r-makarov and @kyralianaka, we think that the most elegant way of making this possible is to add a init_params() method to the Channel:

def init_params(self, states, v, params):
    effective_gNa = 2.3 ** ((params["temperature"] - 37) / 10) * params["gNa"]
    return {"gNa": effective_gNa}

Just like init_state(), init_params does not have to return all parameters, only the ones that are to be modified.

[jnsbck]

I like the idea! Also should allow module wide sharing of these params via make_trainable (if I am not mistaken). Might make sense to implement this in conjunction with #474.