Generalized ion names (#749)

* Replace ionKind enumerations with corresponding string values.
* Rename `ion.hpp` as `ion_info.hpp`.
* Re-jig 'nonspecific' current handling in modcc: make explicit the external source of the corresponding current variable, and treat as non-indexed those locals with an external variable that has no data source, as opposed to special-casing the `ionKind::nonspecific` value.
* Add another field in ion dependency information that captures a write to an ionic reversal potential.

Fixes #748.