add a new phase to the simulator. Basically the simulator now goes

add a new phase to the simulator.  Basically the simulator now goes
through the following phases.

1) Construct all param contexts
2) Call the checkParams() on each context
3) Build the configuration hierarchy
4) Construct all SimObjects
5) Initialize all SimObjects by calling init() on each one
6) Unserialize the checkpoint
7) Register all statisitcs
8) Check validity of all statistics (after that, no new stats)
9) Reset all stats.
10) Call SimStartup() which calls startup() on all SimObjects,
ParamContexts, and any other object deriving from StartupCallback

SConscript:
    no more SimInit() we have SimStartup() now
sim/param.hh:
    Make all params have a startup callback.
sim/sim_events.cc:
    the init callbacks no longer exist.  We can simplify code by
    using startup().
sim/sim_object.hh:
    Make all SimObjects derive from StartupCallback

--HG--
extra : convert_revision : ab81e259eb5510cc597f7bacb2bfb619fb4cc15f