Introduce a ion mechanism

[sg-s]

What we want:

• the ability of channels to conduct specific ions, so that there is some information about what ions flow through channels
• generalizabiity of the calcium_mech, which currently uses the special i_Ca and is_calcium members

Plan

• Add permeability info to all conductances
• CalciumMech should use the permeability structs in the calcium channels
• a new ion mechanism
• pumps, buffers, etc.

Ideally, something that can be used for any ion and be dynamically bound to some ion