ENH: Added new Joule Heating fvOption and test case

    Evolves an electrical potential equation

    \f[
        \grad \left( \sigma \grad V \right)
    \f]

    where \f$ V \f$ is electrical potential and \f$\sigma\f$ is the
    electrical current

    To provide a Joule heating contribution according to:

    Differential form of Joule heating - power per unit volume:

    \f[
        \frac{d(P)}{d(V)} = J \cdot E
    \f]

    where \f$ J \f$ is the current density and \f$ E \f$ the electric
field.
    If no magnetic field is present:

    \f[
        J = \sigma E
    \f]

    The electric field given by

    \f[
        E = \grad V
    \f]

    Therefore:

    \f[
        \frac{d(P)}{d(V)} = J \cdot E
                          = (sigma E) \cdot E
                          = (sigma \grad V) \cdot \grad V
    \f]

Usage
    Isotropic (scalar) electrical conductivity
    \verbatim
    jouleHeatingSourceCoeffs
    {
        anisotropicElectricalConductivity no;

        // Optionally specify the conductiv...