Calculation of the derivatives for snow liquid fluxes

[ougx]

Inaccurate derivatives for snow liquid fluxes are currently used in the snowLiqFlx subroutine for simplicity. The derivative iLayerLiqFluxSnowDeriv needs to be updated in the future (see Line 140 of snowLiqFlx.f90).

[bartnijssen]

@luckymichael : Can you make this a bit more specific, i.e. which routines, which lines, etc. Keep in mind that someone different may pick this up a year from now, so we need to be more descriptive in these issues. The price to pay for working on the code with many people.

[ougx]

@bartnijssen: The location of this issue has been updated. Thanks.