Cryptic/useless names in XIOS messages

[hiker]

BY design, XIOS does not inherit the id. So in case of:

      <file id="output" name="hello_world" output_freq="2s">
        <field field_ref="field1" operation="instant" />
      </file>

If this field needs to be referenced in an message, a automatically created ID (field_undef_id1) is used, which does not help the user to identify which object is referred to. This patch adds a reference name if available, e.g. field_undef_id1_ref=field1, which can help identifying where a problem originates from.