Implement an ElectricConductivity object owned by RunawayFluid

[Embreus]

In order to keep RunawayFluid clean, we should move all functions related to the conductivity calculation into its own object, which could be owned by RunawayFluid.

[Embreus]

The only functions that RunawayFluid should retain are then

        const real_t GetElectricConductivity(len_t ir) const;
        const real_t* GetElectricConductivity() const;
        real_t evaluatePartialContributionConductivity(len_t ir, len_t derivId, len_t n);

whereas the remaining conductivity-related functions could be private methods in the new class.

Possibly we should move the FractionOfBraamsConductivity function from PredictedOhmicCurrentFromDistributionTerm to the new class as well, possibly as a public static method.

[Embreus]

Also, we could probably move the ElectricConductivity object from RunawayFluid to EquationSystem? It really has nothing to do with the runaway rates.