Split gro I/O routines from confio.cpp

Move routines to do gro-format I/O to a separate file.  As with g96io.h,
this would ideally be internal to the fileio module, but unfortunately a
few tools peek into the details of some specific format.

The naming of the routines will be changed separately.

Change-Id: Ia0b190a95f4fb2cd223a6581531aaf45e99d3440