Do not register parameter set in a constructor

There is a public constructor of ParameterSet that registers the constructed parameter set.  This complicates thread safety, because it is hard to find instances of this particular constructor.
This pull request makes this constructor private, and removes the registration from this constructor.   It adds a new public static function of ParameterSet (registerfromString) that invokes this constructor and registers the constructed parameter set.
The only place in CMSSW where this constructor is called is modified to call the new static function.
The new static function has the same thread safety issues as the constructor had previously, but it is much easier to find, and much less likely to be used inappropriately.