Refactor ISF_Acts packages; support only SimKernelMT for ActsFatras

As we will use Acts-Fatras exclusively for the Inner Detector, only
a single simulator/processor is needed.
We are not supporting SimKernel algorithms with
Acts-Fatras tools, so ISF_ActsFatrasServices/ActsSimTool codes were
moved into ISF_ActsFatrasTools/ActsFatrasSimTool, the latter
becoming a BaseSimulatorTool that also implements the processing of
particles.
Supporting only SimKernelMT, ISF_ActsFatrasServices has been removed
altogether.

Other general clean-up of comments and formatting.