Fix bugs in AATSR reflectance correction

There were a couple of bugs in the AATSR
drift correction methods, which meant the
reflectances were systematically different
to the DIMITRI data:

- When removing the drift correction already
  applied, we check the image's metadata for
  the processing file name to check what date
  it was processed. We were using the "GC1"
  file when it should have been the "VC1" file.

- There was a bug in the calculation of the
  number of days since launch, which had T0-T
  instead of T-T0, leading to incorrect values
  in the drift calculation

With these bugs fixed, the resulting reflectance is
v similar to that from DIMITRI (can't expect it to
be indentical due to differences in region extraction)
Fix bug in MERIS correction

There was a bug in the MERIS radiance
correction, with a multpilication instead
of division. This meant that instead of
correcting for solar flux we were adding
an extra seaonal cycle.

Corrected outputs now closely match the
DIMITRI values.
Merge buxfixes from branch 'add_images'