Energy leak fixes in ZM

[crterai]

This fix by Xiaoliang Song plugs two energy leaks. The description, as provided by Xiaoliang:

1) flxsnow(i,k+1) = max(flxsnow(i,k+1), 0._r8) in the subroutine zm_conv_evap () 
This code is part of v2 and the current master. It may result in mass and energy leak when flxsnow <0.
In ZM microphysics, the overly strong snow detrainment may result in negative flxsnow. I have implemented 
an adjustment of snow detrainment in the microphysics code to fix this issue.

2) In the downdraft, the energy change related to ice-phase precipitation evaporation is not fully considered. 
Since the current convective microphysics scheme is designed for convective updraft, the representation of 
microphysical processes in downdraft is very crude. I have removed two line of code related to the change of 
sprd to maintain energy conservation. Since the total precipitation and evaporation is determined by rprd and evp,
this change should not have a noticeable impact on the simulation.