#100 Marshalling/Unmarshalling functions do not allow to add datasets later

[Bernd Löhr]

The marshalling module as implemented currently, is only usable on initialization. Adding dataset descriptions later on is not possible (because of the use of statics).
tau_initMarshalling() should be changed to be able to be called to add more datasets or there should be a separate addDSConfig function.

[Bernd Löhr]

tau_initMarshall() can be called at any time, former dataset entries will be overwritten - the caller needs to combine the entries. -> no change in code needed.