Use Nernst equation to calculate reversal potentials (#387)

Replace constant values for ion species reversal potentials taken from HH model with values that are updated on each time step via the Nernst equation.

* Implement Nernst equation in multicore and gpu back-ends.
* Extend interface of `ion` type to proved a method `update_reversal_potential` which calls the back-end nernst routine.
* Add `valency` and default concentration values to the ion species.
* Add interface for resetting ion species state (for restarting simulations).

Fixes #376