Changed behaviour of start_step=0 to avoid confusion in average compu…

…tation

README.md

@@ -858,7 +858,7 @@ GRIB File: contains data starting from step 0 to 72 every 6 hours: 0,6,12,18,24,
 
   Day 2: same as Day 1 starting from (24+24)-24=24: the step 24 will have a weight of 3, while steps 30,36 and 42 will be counted 6 times, and finally the step 48 will have a weight of 3. 
 
-- if start_step is zero or is not specified, the aggregation will start from 0
+- if start_step is zero or is not specified, the aggregation will behave exactly as using start_step=24, so it will start from 1
 
 ### Accumulation
 For precipitation values, accumulation over 6 or 24 hours is often performed. Here's an example of configuration and execution output in DEBUG mode.

src/pyg2p/main/manipulation/aggregator.py

@@ -167,13 +167,13 @@ def _average(self, values):
             v_ord = collections.OrderedDict(sorted(dict((k.end_step, v_) for (k, v_) in values.items()).items(), key=lambda k: k))
             if self._start == 0 or self._second_t_res:
                 iter_start = self._start
-                iter_end = self._end - self._aggregation_step + 2
+                iter_end = self._end - self._aggregation_step + 2       # CR: TODO: check if +2 instead of +1 is actually needed
             else:
                 iter_start = self._start - self._aggregation_step
                 iter_end = self._end - self._aggregation_step + 1
 
             for iter_ in range(iter_start, iter_end, self._aggregation_step):
-                if self._aggregation_halfweights or self._start == 0:
+                if self._aggregation_halfweights:
                     iter_from = iter_ 
                     iter_to = iter_ + self._aggregation_step + 1
                 else: