Lookup of parameters in LoggingParameterTree seems inconsistent.

There might be reason for it, but that is not obvious to me.

• hasKeyInGroup with an empty groupPrefix will neglect defaultParameters_ as it returns hasKey(key) (which only searches in params_
• hasKeyInGroup with a nonempty groupPrefix will search also defaultParameters_ but only in group groupPrefix. When searching the parent groups it will not.

What behavior is intended here exactly?