Pull request #683: CAS-13766
Merge in CASA/casa6 from CAS-13766 to master
* commit '1917e7b9dad2f4a87410cf8000edf4e5085d24dc':
Minor improvements in the code based on the code review suggestions
fix up error message for purists
Fix a bug related to test failures
Catch unphysical spix values
Add a check for the spectral index if it is physically reasonable