Added function to alphabetize xarray datasets

Variables within an xarray datasets are not guaranteed
to be in any particular dataset. This can be problematic
for datasets with many variables. This commit adds a
function to alphabetize the variable names in a dataset.

src/momlevel/util.py

@@ -11,6 +11,7 @@
 from momlevel import trend
 
 __all__ = [
+    "alphabetize_dataset",
     "annual_average",
     "annual_cycle",
     "default_coords",
@@ -28,6 +29,23 @@
 ]
 
 
+def alphabetize_dataset(ds):
+    """This function reorders the variables in an xarray dataset
+    according to alphabetical order
+
+    Parameters
+    ----------
+    ds : xarray.core.dataset.Dataset
+        Input xarray datset
+
+    Returns
+    -------
+    xarray.core.dataset.Dataset
+    """
+
+    return ds[sorted(ds.variables.keys())]
+
+
 def annual_average(xobj, tcoord="time"):
     """Function to calculate annual averages