Don't install headers depending on GMX_INTERNAL_XDR
There should be no need to install xdrf.h; if some user wants to use the
API, some higher-level API is probably better suitable. With some
reorganization of the headers, all cases where the header was included
from some other installed header were actually unnecessary:
- gmx_fio_getxdr() is now in xdrf.h (somewhat ugly, but so are the
alternatives).
- xdr_datatype is mainly used for edr reading (also for checkpoint
precision), but put it into a separate header for now that can be
installed. It is actually completely independent of the other stuff
in xdrf.h.
Change-Id: I74a66388a8c09cbf4eb9b7b44d7cd13be2776e9c