Extend GBU background test to all wavelengths

[detoma]

We want to extend the GBU test to each individual wavelength.

This test reads the median background from the header, so we need to do this computations per wavelength and store it in the extension.

The changes we will make are the following:

• change the mask used slightly to extend the annulus from 1.1 to 1.5 Rsun and add the post masking
• make the computation for each wavelength - we use the I offband - and store in the extension header
• in the primary header we record he mean of the median background
• in the GBU summary file we report the worst case scenario, i.e.
  if one extension or more have a bkg above the maximum will get a score of 4
  if one extension or more have a bkg below the minimum will get a score of 8