A more detailed tutorial on how to write channel models

[michaeldeistler]

• channel models should not modify their state
• parameters that depend on other parameters should be considered as states whose values are defined in init_state (and are either not returned by update_states at all or it just returns the current value)
• celsius, reversal potential, and so on, are currently considered as channel_params and must defined in every channel
• users should use params and states in compute_current and update_states, and not the global self.channel_params or self.channel_states
• explain current_name and ion_name
• describe that params should never be set with a complicated function inside of self.channel_params = {...}