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naiad-tile command in details
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The ``naiad-tile`` commandline tool allows to extract the metadata from a granule
file at global level (properties or attributes valid for the whole file content)
and at a finer division level (subsets of the granule refered to as **tiles**).

These properties are later registered into an index (implemented by ElasticSearch).
These properties include:

* time frame of the data
* spatial boundaries (shape) of the observed area
* any custom property that one may want to extract from the data, as a future
  search/selection criterium.

The most basic behaviour is to call ``naiad-tile`` with the two mandatory arguments:

.. code-block:: bash

  naiad-tile <file> <mapper>

where:

* ``file`` is the full path to the granule file
* ``mapper`` is the name of the mapper class in **cerbere** package to read this type of
  granule.


for instance:

.. code-block:: bash

  naiad-tile 20100123-EUR-L2P_GHRSST-SSTsubskin-AVHRR_METOP_A-eumetsat_sstmgr_metop02_20100123_000103-v01.7-fv01.0.nc GHRSSTNCFile

This will result in a single tile matching the full area covered by the granule. You see that
by using the ``--show`` option:

.. code-block:: bash

  naiad-tile --show 20100123-EUR-L2P_GHRSST-SSTsubskin-AVHRR_METOP_A-eumetsat_sstmgr_metop02_20100123_000103-v01.7-fv01.0.nc GHRSSTNCFile

This is not exactly what we want, because tiles are meant for fine spatial and temporal subsetting.
A good compromise is usually to have a tile size of about 500km along and accross track. This can
be set by using the ``--xstep`` and ``--ystep`` (along-track) options where the value is given in
pixels. Adjust this value so that the tile size is about 500km.

.. code-block:: bash

  naiad-tile --show --xstep=540 --ystep=540 20100123-EUR-L2P_GHRSST-SSTsubskin-AVHRR_METOP_A-eumetsat_sstmgr_metop02_20100123_000103-v01.7-fv01.0.nc GHRSSTNCFile