Implement thermal conductivity models

[acodcha]

Implement thermal conductivity models:

• PhQ::ThermalConductivityModel
• PhQ::ThermalConductivityModel::ConstantIsotropic
• PhQ::ThermalConductivityModel::TemperatureTableIsotropic
• PhQ::ThermalConductivityModel::ConstantOrthotropic
• PhQ::ThermalConductivityModel::TemperatureTableOrthotropic

For the orthotropic models, their constructors take 3 scalars and 2 directions. Modify direction 2 to be perpendicular to direction 1, and calculate direction 3 to be direction 1 cross direction 2. Construct the dyadic tensor from these.

Requires #205