[fluidsystems] Add a 1p adapter fluidsystem for multi-phase fluid systems
Fixes #528 (closed).
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Adds the 1p adapter
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Uses it in the 1p tests
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uses it in the fluid system tests
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other models can incrementally follow this implementation as everything is compatible
- VtkOutputModule doesn't need phaseOffset anymore (once no model uses it anymore)
- fluidsystem phase index is now always 0 and main component index also 0 (to keep the convention the component indices are switched by the adapter if the second/third phase is chosen)