precipitation conversion

[jesusff]

Hi,

a very common conversion for precipitation is from kg/m2/s to mm/day. This currently fails as the water density is required to convert the kg into volume.

One final attempt to convert the units could be included for precipitation variables here:

convertR/R/udConvertGrid.R

Line 86 in 1b0d48c

if (!ud.are.convertible(u1[x], new.units[x])) stop("Non-convertible units \'ud.are.convertible\' returned FALSE)")

Simply by multiplying the units by the specific volume of water (1 l/kg):

> ud.convert(1, "kg/m^2/s", "mm/day")
Error in ud.convert(1, "kg/m^2/s", "mm/day") :
  Units kg/m^2/s and mm/day are not convertible
> ud.convert(1, "kg/m^2/s*l/kg", "mm/day")
[1] 86400