Feature/air gas viscosity new implementation
This changes the gas viscosity for air and will likely influence all tests which use the air component. But the functions implemented now are all more accurate than the old one which is still available currently as 'oldGasViscosity' in case that someone needs it. It is also included since other components like N2 and O2 use the same approach as in 'oldGasViscosity'. They share the same problem as we had it here. It is an approach which can be generalised, but it is unnecessarily inaccurate since we have better approaches specific to these components available.
- Fix all tests (upload new reference files) using an air component as the new viscosity implementation will most likely change the results
- Fix tests in dumux-lecture
Fixes #333 (closed).