Make all gates staticmethods of the channel classes

[michaeldeistler]

Allow initializing channels at their steady state.

• Moves all gate functions into the class itself (e.g. m_gate() is now within the Na channel)
• Introduce init_state() method for all channels
• remove NaK channel of Pospischil; use Na and K separately

This PR allows to initialize all channels in their steady state:

cell = jx.Cell(...)
cell.set("voltages", -60.0)
cell.init_states()