Introduce MTJ Handle in real life example of converter
This MR introduces MutableMultiTrajectoryHandle into the existing TrkToActsConvertorAlg. This simplifies declaration of all the backends significannnly.
Following are all changes needed to get this working.
In addition, the lifetime of the backends in MutableMTJ and ConstMTJ became clear and simplificaitons to the construction API of it are part of this MR.
In summary: the MutableMTJ when crated owns the backends.
Once filles the ConstMTJ is constructed out of it using MulableMultiTrajecotryHandle which in addition takes care of moving ownership from MutableMTJ instance to SG. Besides the SG interation this is cheap operation.
The ConstMTJ can be registered in SG for client.