Move gstat.h to gmxana/

Move gstat.h to gmxana/

Remove unnecessary inclusions of gstat.h from outside gmxana/.
Now that the header is no longer used outside gmxana/, move it into that
directory.  Still install the header, since it contains a wide mixture
of stuff, some of which may actually be used by user tools.

This cleans up the include dependency graph considerably, since the
legacyheaders/ directory no longer depends on commandline/.  This was a
source of a cyclic dependency that made the graph layout very confusing.

Change-Id: I3843f2773d6b393e4d260e84478ab559ab8e00d7