Enhance configuration to specify how a transformation is triggered.

[clementval]

Currently, transformation are triggered by directive or hardcoded when they have to be triggered for each translation unit.

The idea would be to set this information in the configuration file.

<claw version="0.3">
  <global>
  </global>
  <groups>
    <group type="independent" name="internal-xcodeml" trigger="translation_unit" class="cx2x.translator.transformation.utility.XcodeMLWorkaround" />
    <group type="independent" name="parallelize-forward" trigger="directive" class="cx2x.translator.transformation.claw.parallelize.ParallelizeForward" />
  </groups>
</claw>

[clementval]

Implemented by #195