This is quite a hack: Even though energy is not a "phase" and it is
also not considered in MaxNumPhases and pu.num_phases because this
would break a lot of assumptions in old code, it is nevertheless
assigned an "canonical index" that can be translated "active index"
via PhaseUsage::phase_pos[]. This awkwardness is needed because much
of the legacy OPM code conflates the concepts of "fluid phase" and
"conserved quantity" and fixing that issue would basically mean an
almost complete rewrite of much of the legacy code. That said, the
same statement applies to polymer and solvent, but these are currently
handled as even more second-class citizens because they are not even
given a canonical index and also cannot be translated into an active
one.
This is intentionally black-oil specific because we presently do no
know how to handle other cases (e.g., more phases or number of phases
different from number of components).
