Changed handling of blank environment, fixed section handling on command line, cleaned up testing, added release changes

* Changed behavior if environment name is set but empty: now uses default instead of looking for '' environment
* Fixed command line handling: different variables in the same section are now all recognized (see IssueID #5501). The test cases are updated accordingly
* Re-organized test cases to group command line handling in its own class
* Added empty [namelists] section to test config, simplifying test writing
* Added CHANGES.txt for upcoming release 0.3.0, updated setup to 0.3.0dev