Adds functions for models to compute total energy and water volume(mass) per area

[kmdeck]

Purpose

Add helper functions which compute the total water volume and energy per unit area on land.
partially closes Issue #1038

To-do

• Follow up PR - add conservation test
• Follow up PR: fix Remove allocations in energy hydrology total water content computation #1078

Content

Creates the total_energy_per_area! and total_liq_water_vol_per_area! functions for AbstractModels
Creates the specific method of these for: Richards, EnergyHydrology, Snow, Canopy
Tests these methods for standalone models
Creates a method for LandModels
Tests this method for the snowy land model

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• I have read and checked the items on the review checklist.

[juliasloan25]

This looks great, thank you Kat! It would be helpful to clarify that, although total_liq_water_vol_per_area! returns the volume of liquid water, it includes contributions from all phases.

Could you also update the NEWS please?

[kmdeck]

This looks great, thank you Kat! It would be helpful to clarify that, although total_liq_water_vol_per_area! returns the volume of liquid water, it includes contributions from all phases.

Could you also update the NEWS please?

yes! will do for both