Print a warning rather than failing if we're unserializing and there's an

Print a warning rather than failing if we're unserializing and there's an
object in the config that was not in the checkpointed config.
This code compiles, but I haven't tested it... I'm committing it so Ron
can have it.  Should not effect anything that currently works.

base/inifile.cc:
base/inifile.hh:
    Add sectionExists() method so you can query whether a section exists
    without knowing any of the entry names that would be in it.
sim/serialize.cc:
sim/serialize.hh:
    Add Checkpoint::sectionExists() (pass through to IniFile).

--HG--
extra : convert_revision : 905db122afdfe55946ab8493ccac0b1e715bc7c6