Return usage.get_{monthstat,daystat} in expected format (#394)

* Basic fix for issue: https://github.com/python-kasa/python-kasa/issues/373
Change usage module get_daystat and get_monthat to return dictionaries of date index: time values as spec'd instead of raw usage data. Output matches emeter module get_daystat and get_monthstat

* Fixed some formatting and lint warnings to comply with black/flake8
Use the new _convert function in emeter for all conversions rather than the one in smartdevice.py
Removed unused function _emeter_convert_emeter_data from smartdevice.py

* Added a first pass test module for testing the new usage conversion function

* Changes based on PR feedback
Tidied up some doc string comments
Added a check for explicit values from conversion function

* Rebase on top of current master, fix docstrings

---------

Co-authored-by: Teemu Rytilahti <tpr@iki.fi>

kasa/modules/usage.py

@@ -1,5 +1,6 @@
 """Implementation of the usage interface."""
 from datetime import datetime
+from typing import Dict
 
 from .module import Module, merge
 
@@ -50,8 +51,8 @@ class Usage(Module):
 
         return converted.pop()
 
-    async def get_daystat(self, *, year=None, month=None):
-        """Return daily stats for the given year & month."""
+    async def get_raw_daystat(self, *, year=None, month=None) -> Dict:
+        """Return raw daily stats for the given year & month."""
         if year is None:
             year = datetime.now().year
         if month is None:
@@ -59,13 +60,45 @@ class Usage(Module):
 
         return await self.call("get_daystat", {"year": year, "month": month})
 
-    async def get_monthstat(self, *, year=None):
-        """Return monthly stats for the given year."""
+    async def get_raw_monthstat(self, *, year=None) -> Dict:
+        """Return raw monthly stats for the given year."""
         if year is None:
             year = datetime.now().year
 
         return await self.call("get_monthstat", {"year": year})
 
+    async def get_daystat(self, *, year=None, month=None) -> Dict:
+        """Return daily stats for the given year & month as a dictionary of {day: time, ...}."""
+        data = await self.get_raw_daystat(year=year, month=month)
+        data = self._convert_stat_data(data["day_list"], entry_key="day")
+        return data
+
+    async def get_monthstat(self, *, year=None) -> Dict:
+        """Return monthly stats for the given year as a dictionary of {month: time, ...}."""
+        data = await self.get_raw_monthstat(year=year)
+        data = self._convert_stat_data(data["month_list"], entry_key="month")
+        return data
+
     async def erase_stats(self):
         """Erase all stats."""
         return await self.call("erase_runtime_stat")
+
+    def _convert_stat_data(self, data, entry_key) -> Dict:
+        """Return usage information keyed with the day/month.
+
+        The incoming data is a list of dictionaries::
+
+               [{'year':      int,
+                 'month':     int,
+                 'day':       int,     <-- for get_daystat not get_monthstat
+                 'time':      int,     <-- for usage (mins)
+               }, ...]
+
+        :return: return a dictionary keyed by day or month with time as the value.
+        """
+        if not data:
+            return {}
+
+        data = {entry[entry_key]: entry["time"] for entry in data}
+
+        return data