ParametersParser refacor

WIP
Note on _where_ to check for this warning:

Either there or once the parameter is processed completely – I was thinking to add some finalize() function that would be called at the end of `SectionParser::parse()` to do final checks similar to this one. What do you think? 

The reason for separate "finalize" call against handling it in nested sections is that in theory you might handle multiple nested `ValuesSectionType` section in which case you would want to run the check once you populate all the values...