This subroutine does a number of things invoving the time
values in every FITS HDU for a list of file types.
If an extension had TSTART and TSTOP keywords, it checks to
be sure that neither is zero and that TSTART <= TSTOP.
It also sets the values for the DATE-OBS, TIME-OBS, DATE-END,
TIME-END keywords. Finally it keeps a running tally of the
earliest TSTART and the latest TSTOP. These values are used
to mark start and finish of the entire observation.
Every TIME column is checked for whether it is in order.
The BAT bcdh files are not checked since the BAT_TELEM column
is not supposed to be in order.
Finally, for every GTI extension, the subroutine checks that
no two GTIs overlap, that they are in order and that they
all have STOP > START.