Fix a problem in iraf.imstat

For example:

------------------------------

ccdred> imstat 01ZERO.fits

# IMAGE      NPIX      MEAN    STDDEV      SKEW  KURTOSIS       MIN       MAX

Error reading image 01ZERO.fits ...

------------------------------

There is already a 01ZERO.fits, but if we calculate a new 01ZERO.fits by using the same method, the "imstat" command will be wrong in reading the 01ZERO.fits.

So you need to delete the previous 01ZERO.fits file.

-----------------------------

ccdred> imstat 01ZERO.fits

#  IMAGE      NPIX      MEAN    STDDEV      SKEW  KURTOSIS       MIN       MAX

    01ZERO.fits   4194304     505.6    0.6513     -31.9     26.12     492.9     512.7

_________Anbing Ren @BNU   2016-08-17 22:12