Config class improvements (#826)

* Subclass ConfigParser
* Remove redundant variables and update tests
* Update file description
* Move NWConfigParser class to common.py
* Improve coverage of config class
* Use NWConfigParser also in themes.py
* Update config class file header
* Fix comment
* Update the usage of ConfigParser to recommended practice and extend tests
* Improve logging for info level a bit

tests/conftest.py

@@ -91,10 +91,7 @@ def fncDir(tmpDir):
         shutil.rmtree(fncDir)
     if not os.path.isdir(fncDir):
         os.mkdir(fncDir)
-    yield fncDir
-    if os.path.isdir(fncDir):
-        shutil.rmtree(fncDir)
-    return
+    return fncDir
 
 
 @pytest.fixture(scope="function")