Add conductivity to implicit voltage solve step for stability. (#735)

Changes implicit solve step from:
> solve (c/δt + L) v' = c/δt v - J

to
> solve (c/δt + g + L)v' = (c/δt +g) v - J

where _c_ is capacitance, _g_ is membrane conductance, _J_ is membrane current.

* Compute conductivity contribution for mechanisms from symbolic d/dv of current contribution (extracted from linearity test.)
* Add new modcc 'source kind' for conductivity; tie to `vec_g_`.
* Add conductivity field to fvm shared state.
* Include conductivity in matrix assemblies for solution.

Fixes #633.