Turbulent fluxes above the canopy shuld be NaN, not zero

[vgro]

Is your feature request related to a problem? Please describe.
The turbulent fluxes for the above canopy layer are currently initialized with zeros and then not further updated. However, they should really be NaN.

Describe the solution you'd like
Set to NaN in setup and check that all functions that use the variables still work properly.