TWP/SMET/C3 - Negative rainfall rates

Due to limitations of the Zeno dataloggers all voltage measurements
from the optical rain gauge (ORG) were converted to rainfall rates. 
This resulted in negative values for rain rates.  Any value below
0.10 mm/hr should be considered to be 0 mm/hr.  New dataloggers 
alleviated this problem.

Consider rainfall rate data less than 0.10 mm/hr as 0.0 mm/hr

• precip_mean
• precip_sd
• precip_max
• precip_min