Improvement to statistics.py module

[andersy005]

Checklist

• Enable and install pre-commit to ensure style-guides and code checks are followed.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

• Fixes statistics.weighted_mean() doesn't work with dim=None #137
• Refactors test_statistics.py

[mnlevy1981]

I verified that this PR lets me drop the dim= argument to esmlab.statistics.weighted_mean():

$ git diff marbl_diags
diff --git a/marbl_diags/analysis_ops.py b/marbl_diags/analysis_ops.py
index cd6f8d6..506053e 100644
--- a/marbl_diags/analysis_ops.py
+++ b/marbl_diags/analysis_ops.py
@@ -125,8 +125,8 @@ def _plot_climo(AnalysisElement, valid_time_dims):
 def _compute_stats(field, TAREA):
     fmin = np.nanmin(field)
     fmax = np.nanmax(field)
-    fmean = esmlab.statistics.weighted_mean(field, dim=field.dims, weights=TAREA).load().values
-    fRMS = np.sqrt(esmlab.statistics.weighted_mean(field*field, dim=field.dims, weights=TAREA).load().values)
+    fmean = esmlab.statistics.weighted_mean(field, weights=TAREA).load().values
+    fRMS = np.sqrt(esmlab.statistics.weighted_mean(field*field, weights=TAREA).load().values)

Thanks for updating this!

[andersy005]

Awesome! @mnlevy1981, thank you for confirming that this PR addresses the issue.