Subtract f0 from f24 for snowfall (#460)

Refs: #1

Author: malloryprow

parm/metplus_config/stats/global_det/atmos_grid2grid/PCPCombine_fcstGLOBAL_DET_24hrAccum_snow.conf

@@ -38,7 +38,7 @@ FCST_PCP_COMBINE_IS_DAILY_FILE = False
 FCST_PCP_COMBINE_OUTPUT_ACCUM = 24H
 #FCST_PCP_COMBINE_MIN_FORECAST = {ENV[fhr_start]}
 #FCST_PCP_COMBINE_MAX_FORECAST = {ENV[fhr_end]}
-FCST_PCP_COMBINE_USE_ZERO_ACCUM = False
+FCST_PCP_COMBINE_USE_ZERO_ACCUM = True
 FCST_PCP_COMBINE_DATA_INTERVAL = 24
 FCST_PCP_COMBINE_TIMES_PER_FILE =
 FCST_PCP_COMBINE_OUTPUT_NAME = {FCST_PCP_COMBINE_INPUT_NAMES}_A24