This change is the logical counterpart of SimGear's change from commit
79f869a7f32910197be72b21f6489fbbba02c836 that moved the following files
from simgear/misc to simgear/io/iostreams:
gzcontainerfile.cxx
gzcontainerfile.hxx
gzfstream.cxx (formerly zfstream.cxx)
gzfstream.hxx (formerly zfstream.hxx)
sgstream.cxx
sgstream.hxx
sgstream_test.cxx