Simplify merge method.

Simplify merge method by using list and dictionary comprehensions instead of
nested for loops. This also avoids the need to check if some of the elements
of the reports arguments are None, and the need to check if reports contains
more than one report. This should also be a small performance improvement.
This change also adds a bit more detail to the docstring for the merge method.
The creation of a new empty report is also modified to no longer use an
intermediary list of keys.

Author: alexandermorgan

pvlib/bifacial.py

@@ -159,8 +159,7 @@ def build(report, pvarray):
         back surface of center pvrow (index=1)"""
         # Initialize the report as a dictionary
         if report is None:
-            list_keys = ['total_inc_back', 'total_inc_front']
-            report = {key: [] for key in list_keys}
+            report = {'total_inc_back': [], 'total_inc_front': []}
         # Add elements to the report
         if pvarray is not None:
             pvrow = pvarray.pvrows[1]  # use center pvrow
@@ -177,13 +176,11 @@ def build(report, pvarray):
 
     @staticmethod
     def merge(reports):
-        """Works for dictionary reports"""
-        report = reports[0]
-        # Merge only if more than 1 report
-        if len(reports) > 1:
-            keys_report = list(reports[0].keys())
-            for other_report in reports[1:]:
-                if other_report is not None:
-                    for key in keys_report:
-                        report[key] += other_report[key]
+        """Works for dictionary reports. Merges the reports list of 
+        dictionaries by flattening the lists for each key into a single 
+        super list. Returns a dictionary with two list values."""
+        # Dictionary comprehension obviates the need to check if there are more
+        # than one report, and if one of the elements in reports is None.
+        report = {k:[item for d in reports for item in d[k]]
+                  for k in reports[0].keys()}
         return report