Separate the AATSR directions

We previously had separate database entries for
the two AATSR directions (forward looking and
nadir looking). This meant the geotiff had
2 x nbands arrays in it, but with no way of
labelling which order they come in.

Change so that we create separate geotiff files
(and quicklooks) for each direction, so that
the files are then consistent with other sensors
and have the expected number of arrays inside.

This also means we now use the correct array
of flag values, rather than the using nadir
flags for both directions.
Add AATSR corrections

Following the DIMITRI code, add corrections to
the AATSR reflectance and viewing angles:

- Convert elevation angles (ie zenith==90) to
  zenith angles (ie zenith==0)

- Correct reflectance for solar angle

- Correct band 3 (1600nm) reflectance for non-
  linearity (if required, based on value of GC1
  filename)

- Remove drift correction, and re-apply using
  look up table. (And add this look up table to
  the aux_files directory)

NB these corrections are only applied to the
reflectance wavelengths and not the brightness
temperature ones.
Rebuild docs to include new correction methods