population_template: Fix aggregation weights import

Previous code attempted to write to the dictionary while looping over it; this yields an Exception.

bin/population_template

@@ -558,19 +558,17 @@ def parse_input_files(in_files, mask_files, contrasts, f_agg_weight=None, whites
     import csv  # pylint: disable=import-outside-toplevel
     try:
       with open(f_agg_weight, 'r') as fweights:
-        agg_weights = dict((row[0].lstrip().rstrip(), row[1]) for row in csv.reader(fweights, delimiter=',', quotechar='#'))
+        agg_weights = dict((row[0].strip(), row[1].strip()) for row in csv.reader(fweights, delimiter=',', quotechar='#'))
     except UnicodeDecodeError:
       with open(f_agg_weight, 'r') as fweights:
         reader = csv.reader(fweights.read().decode('utf-8', errors='replace'), delimiter=',', quotechar='#')
-        agg_weights = dict((row[0].lstrip().rstrip(), row[1]) for row in reader)
+        agg_weights = dict((row[0].strip(), row[1].strip()) for row in reader)
     pref = '^' + re.escape(get_common_prefix(list(agg_weights.keys())))
     suff = re.escape(get_common_postfix(list(agg_weights.keys()))) + '$'
-    for key in agg_weights.keys():
-      agg_weights[re.sub(suff, '', re.sub(pref, '', key))] = agg_weights.pop(key).strip()
-
+    agg_weights = {re.sub(suff, '', re.sub(pref, '', key)):agg_weights[key] for key in agg_weights.keys()}
     for inp in inputs:
       if inp.uid not in agg_weights:
-        raise MRtrixError('aggregation weight not found for %s' % inp.uid)
+        raise MRtrixError('aggregation weight not found for input "%s"' % inp.uid)
       inp.aggregation_weight = agg_weights[inp.uid]
     app.console('Using aggregation weights ' + f_agg_weight)
     weights = [float(inp.aggregation_weight) for inp in inputs if inp.aggregation_weight is not None]