BUG: natural heat tranfer coefficient flag value 2 #9

Describe the bug
Flag to select evaluate heat transfer coefficient by natural convection 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 2
5. Save and run the software
6. See error

Expected behavior
Natural convection heat transfer coefficient with the external environment evaluated with a suitable correlation.

Desktop (please complete the following information):

• OS: Windows 10