Follow-up from "Feature/simplify effective laws"

The following discussion from !1605 (merged) should be addressed:

• @DennisGlaeser started a discussion: (+3 comments)

  I thought we removed wPhaseIdx from the Indices and made this a runtime-thing? How does this actually compile?

• Make wetting phase index retrievable from 2p volvars
• Use them in Johansen effective thermal conductivity law