Add new parameterization of snow unloading

[arbennett]

This work with @cassielumbrazo adds a new decision along with a new parameterization. The decision is snowUnload, which will have 2 options. meltDripUnload was the previously default implementation. The new parameterization, windUnload implements a snow unloading scheme described in A new snow cover fraction parametrization for the ECHAM4 GCM eq 13.

Further testing is needed before this should be formally reviewed, but appears to be working as intended:

This feature adds 4 new parameters which need to be specified. They are, along with some default values from Roesch et al:

! Minimum temperature for temperature unloading (T< minTempUnloading causes no unloading) (K)
minTempUnloading          |       270.16 |       260.16 |       273.16
! Minimum wind for wind unloading (v < minWindUnloading causes no unloading) (m/s)
minWindUnloading          |       0.0000 |       0.0000 |       10.000
! Scales the speed at which unloading due to temperature occurs (inversely proportional) (K*s)
rateTempUnloading         |      1.87d+5 |       1.0d+5 |       3.0d+5
! Scales the speed at which unloading due to wind occurs (inversely proportional) (m)
rateWindUnloading         |      1.56d+5 |       1.0d+5 |       3.0d+5

[bartnijssen]

@arbennett : Further testing is needed before this should be formally reviewed

Do you mean that further testing is needed before code review or before this can be reviewed science-wise? Just want to make sure I understand whether you want me to do anything at this time.

[arbennett]

@bartnijssen - Further testing is needed before review of the code. You don't have to do anything yet.

[arbennett]

It appears this is okay to be reviewed now.

[cassielumbrazo]

I have evaluated the algorithm and tested all new parameters at multiple sites. It appears to be working in all tests. An example at Niwot Ridge, CO below.

Another example, testing the minimum wind speed threshold in meters/sec.

I am also updating documentation describing this new decision.

[bartnijssen]

Will be merged before SUMMA V3.0 after completing series of PRs related to changes to decision file, file manager, and some general cleanup