Add extended Richards model with water diffusion in gas phase and possible variable switch

Timo Kochrequested to merge
feature/extendend-richards into next

add extended Richards law with water diffusion in gas phase depends on !528 (merged)

  • Solve how to get diffusion of only one component in one phase
  • Fix fluxvars, see #391 (closed)
Edited by Timo Koch

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