Add averages with time dimension removed (#236)

- Add new `.average()` method to `TemporalAccessor`
- Rename previous `.average()` to `.group_average()`
- Refactor many private methods in `TemporalAccessor` class
  - Rename methods and variables, update docstrings
  - Refactor `._validate_weights()` for a significant performance increase of about 4-5x (for the monthly Dataset I was testing on)
- Remove redundant tests for private methods

Author: tomvothecoder

.vscode/xcdat.code-workspace

@@ -5,6 +5,7 @@
         }
     ],
     "settings": {
-        "autoDocstring.docstringFormat": "numpy"
+        "autoDocstring.docstringFormat": "numpy",
+        "editor.formatOnSave": true
     }
 }

docs/api.rst

@@ -104,6 +104,7 @@ Methods
     Dataset.bounds.add_missing_bounds
     Dataset.spatial.average
     Dataset.temporal.average
+    Dataset.temporal.group_average
     Dataset.temporal.climatology
     Dataset.temporal.departures
     Dataset.temporal.center_times