Implement a more realistic lower layer temperature for OOP

[jrenaud90]

Currently, the base temperature of a layer is equal to its average temperature (layer.temperature_lower = layer.temperature).

In reality, this temperature will be higher. Generally, this is found via an assumed viscosity drop across the layer (See Hussmann & Spohn 2004).

• Look into seeing if this is a decent enough model to use.
• Determine if the viscosity drop is going to be robust enough to handle different viscosity equations (Reference vs. Arrhenius).
• Implement it into the layer class.