all temperature should be in celsius

soil temperature in CF is in K, capful assumes it is in C.

@ankurdesai @mdietze @jam2767 this fixes the gapfilling for soil temperatures..
Just want to promote that the following is more clear / less error prone than conversions that use constants:
  
```r
ud.convert(Extracted[, 'Ts1_f']), "Celsius", "Kelvin")
```