getParameterSetNames is really a const function

I happened to notice that both functions in the ParameterSet::getParameterSetNames overload set are really const functions, but only one of them has const in its signature. This simple pull request adds const to the signature of the other function, which should also have it. This makes one fewer non-const function of class ParameterSet.
To avoid ambiguity, the optional argument of the other function is made mandatory.