fixed errors in core/parameter.hpp

1. forward declaration of 'parameter_set' was implemented twice
2. (const_)parameter_link classes had not unique parameter names
relative to the forward declaration, ie,
parameter_link(parameter_set & parameter_set) :
parameter_set(parameter_set) {} (the forward declaration was again
parameter_set ..)

-> so for three different entities the same name, compiler complained
-> fixed.

also used some_name_ .. underscore postfix for the member variables, as
we agreed in our coding guidelines