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/tests/test_usage.py

@@ -0,0 +1,22 @@
+import pytest
+
+from kasa.modules import Usage
+
+
+def test_usage_convert_stat_data():
+    usage = Usage(None, module="usage")
+
+    test_data = []
+    assert usage._convert_stat_data(test_data, "day") == {}
+
+    test_data = [
+        {"year": 2016, "month": 5, "day": 2, "time": 20},
+        {"year": 2016, "month": 5, "day": 4, "time": 30},
+    ]
+    d = usage._convert_stat_data(test_data, "day")
+    assert len(d) == len(test_data)
+    assert isinstance(d, dict)
+    k, v = d.popitem()
+    assert isinstance(k, int)
+    assert isinstance(v, int)
+    assert k == 4 and v == 30