Modular simulator: Separate trajectory element and signaller

Modular simulator: Separate trajectory element and signaller

The trajectory element implemented both the ISimulatorElement and
the ISignaller interface. It was both signalling its clients that a
trajectory writing step was about to occur during task list creation,
and actually performing trajectory writing during the simulation.

In general, the tasks of signallers and elements, their call site in
the code, and their build and ownership procedures are all significantly
different. The dual nature of the trajectory element makes this
differentiation less clear, and requires some workarounds. Additionally,
separating the trajectory element in separate element and a signaller
requires no changes on client side, since all clients of the trajectory
element with integrated signaller already implemented two interfaces,
ITrajectorySignallerClient and ITrajectoryWriterClient.

The current change hence splits the current trajectory element into
a pure element and a pure signaller. This will make subsequent changes,
especially the introduction of a builder approach much easier. It also
removes support for combined elements and signallers: It renames the setup
method of the ISignaller interface from `signallerSetup` to `setup`, as
this differentiation was only used for classes implementing both element
and signaller interfaces. It also simplifies the handling of signallers
in ModularSimulator, as having a separate ownership and call list is not
needed anymore.

Refs #3437