Allow user to specify variable units

@nperez proposed this as a feature. Opening this issue for any discussion related to this development.

Users would specify the desired units in the recipe, for example:

  Variables:
    name: tas
    units: K
    freq: monthly_mean 

Then the unit conversion, if needed, would be done at the beginning of the workflow after loading the data.

Currently unit conversion is always done for the case of precipitation (see #38 (closed) and #56 (closed)), accompanied by an INFO message. Some special considerations might be necessary for this particular case.