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..
yes, but that makes the code 2-3 times slower:
```
require(udunits2)

t <- sample(-400:400, 20000, replace=TRUE)/10

system.time(replicate(expr=t+273.15, n=10000))
system.time(replicate(expr=ud.convert(t, "Celsius", "Kelvin"), n=10000))
```