File Reading Into Cactus
------------------------

Author: Tom Goodale
Date: January 2001
Status: Initial specification, for discussion

Currently we have overloadable functions for output, but none for
input.  We need something like

int CCTK_ReadVar(CCTK_INT flags, 
                 CCTK_INT *iteration, 
                 CCTK_REAL *time,
                 char *variable,
                 char *filename)
and

int CCTK_ReadVarByMethod(CCTK_INT flags, 
                         CCTK_INT *iteration, 
                         CCTK_REAL *time,
			 char *method,
                         char *variable,
                         char *filename)
 
where flags is a bitset containing
 
ITERATION
TIME
LAST
 
and the actual values for the dataset read are returned.

This allows people to get the last dataset in a file, work out what
iteration it corresponds to, and then work backwards if they want, ditto
with time.  The time could also be used, if we are feeling adventurous, to
allow people to interpolate to get data at a particular time.

Maybe need to add an alias variable, so can read a variable into a different
names grid variable.

Maybe need to allow the reading in of more that one variable.