write forecast reference T0

Design Review - Forecast Reference Field output:

This proposal would add functionality to XIOS to allow users to opt in to writing out the T0 initialisation timestep data into output files.

This is common practice for NWP models and many data usage systems currently receive data files that include the T0 reference fields as part of the forecast output.

it is proposed that a simple boolean is added to the XML configuration, such as:

<file_definition type="one_file">
  <file id="axis_output" output_freq="2ts" write_initial_condition=".TRUE.">
      <field field_ref="pressure" operation="average"/>
  </file>
</file_definition>

The default value for write_initial_condition would be False, and it is proposed to affect all fields in the file.

To ease implementation, then is may be useful to consider the case where write_initial_condition is True, but no data is sent at timestep 0. in a prototype implementation it seemed simple to handle this case without XIOS throwing an error. Thus, both the boolean and the send_field would be required to provide data in the output file.

An initial assessment of risk to the code base suggests that this is a fairly isolated change:

• extending the user interface (XML & Fortran) to allow for the optional use of a new attribute
• adapting node/field node/file & filter/temporal_filter to manage the T0 case

(this ticket is migrated from Trac: https://forge.ipsl.fr/ioserver/ticket/208)