pyIRSF-2.1.1 released

Making badpixel masks failed when only sky-bias frames with low background levels,
~ a few x 10 ADUs, were available. This has been fixed.

• irsf.normalize_and_settozero_median_sky(para.band, skyarr) was replaced
  with sky4mask = irsf.measure_settozero_mediansky(para.band, skyarr)
  in sirius.py and sirpol.py

• irsf.measure_settozero_mediansky() was created.

• irsf.mkbadmask2() was modified.

  A badpixel mask is created from the sky bias frames available in the obslog file.
  All the sky bias frames are normalized and combined to a sigle image, of which
  median is about 1.0. Pixles less than 0.5 or greater than 2 are considered as the
  bad pixels in the normalized and combined image.
  When, for example, ND filters and/or very short exposure times, like 1.6 sec, are
  used, the median of the background level is almost zero and the normalization fails
  to pick up bad pixels.
  When all the sky bias frame for a band has the median less than 90 ADU, the flat
  image is used for the bad pixel selection.
  When you use the flat images bungled with the pyIRSF package, using the flat images
  in making the mask may results in insufficient coverage of bad masks,
  because the number of bad pixels on the SIRIUS arrays have been increasing year by year.