Kwargs with None need to be passed using 'null' in yml files

[JoostBuitink]

Currently, when passing None to a kwargs, the yml parser does not convert this to a Python None object. Instead, null should be specified in the yml file.

One possible solution is to accept for None in the yml file, and replace those with null before parsing to the yml reader. This would also mean that null values should be replaced with None before writing to a yml file.