Handle none in toml or no value defined for a parameter during save

[tjadevries]

Actually two problems that can be solved together:

• in toml, there is no way to save a None value (see e.g. Add a none type toml-lang/toml#921 ); we need a way to handle a config where a parameter has value None
• we are going to provide a convenience function to save a parameter file to facilitate the user (especially useful for config); if a parameter does not have a default value, then it's section cannot be instantiated and yet we would like to be able to save a file to start with

Solution: add a comment line/section that states either parameter = None or parameter =