config: Add SubSystem container for simobjects

config: Add SubSystem container for simobjects

This patch adds the SubSystem container for grouping
simobjects together in logical subsystems to facilitate
building a larger system from constituent parts.  The container
is simply a non-abstract empty simobject to hold the components
that will be connected as its children.  In simulation the
object does not participate, its only use is during configuration
of the system.