BUG: natural heat tranfer coefficient flag value 4

[DanielePlacidoPolito]

Describe the bug
Flag to select evaluate heat transfer coefficient by natural convection and radiation with the environment using a suitable correlation does not work.

To Reproduce
Steps to reproduce the behavior:

1. Go to file *coupling.xlsx
2. Click on sheet HTC_choice
3. In the matrix, select an allowed interface with the external environment
4. Set the flag to 4
5. Save and run the software
6. See error

Expected behavior
Total heat transfer coefficient with the environment evaluated as the sum of the natural convection heat transfer coefficient (evaluated with a suitable correlation) and the radiative heat transfer coefficient.

Desktop (please complete the following information):

• OS: Windows 10