Snow to runoff conversion if snow depth exceeds 10m

[mee067]

This feature comes with CLASS 3.6.2 to prevent over growth of snow on ice sheets as per Diana (verbal communication with Dan - maybe?)

A code block was initially added to CLASSW.f`to do this with a hard-coded threshold of 10m for Zsnow.

I moved this code block to RUNCLASS36_module.f90 to be done after blowing snow (PBSM) is called if active.

This feature helped avoid snow energy/water balance crashes in high ground regions (non-glacier tiles) where snow kept accumulating unchecked but it caused runoff to occur randomly in mid-winter from those tiles giving unreasonable spikes in the hydrographs. Different tiles in different locations would reach this threshold at different times. It is unrealistic to just send the excess snow to runoff, just to get rid of it irrespective of whether conditions are correct to melt it.

I also tested different values of that threshold (10, 20, 30, 40, and 50m) which reduced the spikes but it is still unrealistic.

I recommend to either:

1. Disable this feature altogether
2. Keep it but add a flag to activate it if desired. In such case, the threshold depth should be an exposed parameter.