check water conservation (incl. snow)

[LenkaNovak]

We need to check the water conservation logger (incl. snow), and apply appropriate allowable bounds.

Components

• fix conservation checker / plotting
  • note that now Atmos uses the kg/m^2/s for evaporation, and m/s for precipitation!
• add snow - small errors remain - see Fix land snow #421
• generalize ConservationChecker, to allow for more general tests and extensible development (drop the dependency on underlying models' data structures) Revamp ConservationChecker #436
• add unit test for water
• apply allowable bounds to unit tests
• remove the usage of limiters, which cause worse water conservation, see below (atmos default also doesn't use them)

QA

• unit tests pass to FP precision
• no change in performance
• slabplanet conserves water and energy (though not to FP precision - see above)

Note, if we apply limiters, water conservation breaks: