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.

Note comments are retained for future iterations on this work.