revisit implementation of soil temperature

[vgro]

The soil temperature calculation needs to be checked. Then give either a citation to or writing out the temperature function that is being differentiated in the docs.

[hrlai]

Not sure if this is related, but surface and soil temperatures are very low in ve_example. I understand that example data are not to be taken seriously, but is it expected / intended?

Canopy top temperature are fine:

But surface temperature is sub-zero:

And topsoil temperature simply reflects surface temperature with some positive shift upward: