Move IOStreams-related files to simgear/io/iostreams; rename zfstream.[ch]xx to gzfstream.[ch]xx

- Rename zfstream.cxx (resp. zfstream.hxx) to gzfstream.cxx (resp.
  gzfstream.hxx)

  This is because these files only deal with the gzip format (RFC 1952),
  while zlib can actually read and write two slightly different formats:
  this one and the "ZLIB Compressed Data Format" (RFC 1950). Since I am
  going to add std::streambuf and std::istream subclasses able to deal
  with both formats (and supporting data sources that are general
  std::istream instances, not just files), this renaming will make
  things a bit clearer, I hope.

- Add new folder simgear/io/iostreams and move the following files to
  this folder:

    simgear/misc/gzcontainerfile.cxx
    simgear/misc/gzcontainerfile.hxx
    simgear/misc/gzfstream.cxx
    simgear/misc/gzfstream.hxx
    simgear/misc/sgstream.cxx
    simgear/misc/sgstream.hxx
    simgear/misc/sgstream_test.cxx

- Adapt other files accordingly (mainly #includes and CMakeLists.txt
  files).

simgear/math/interpolater.cxx

@@ -31,7 +31,7 @@
 #include <string>
 
 #include <simgear/debug/logstream.hxx>
-#include <simgear/misc/sgstream.hxx>
+#include <simgear/io/iostreams/sgstream.hxx>
 #include <simgear/misc/sg_path.hxx>
 #include <simgear/props/props.hxx>