ENH: heatTransferCoeff: add optional Nusselt number functionality

  DOC: heatTransferCoeff models: complete remaining header docs
  STYLE: heatTransferCoeff models: use auto specifier when appropriate

  Optionally, the Nusselt number (i.e. the ratio of convective to conductive
  heat transfer at a boundary in a fluid) can be output:

  ```math
      Nu = \frac{h L}{\kappa}
  ```

  where
  ```
    Nu | Nusselt number
    h  | Convective heat transfer coefficient of the flow
    L  | Characteristic length that defines the scale of the physical system
    \kappa | Thermal conductivity of the fluid
  ```