Resolve "implement lazy data preprocessing"

Closes #292 (closed)

mlair/helpers/statistics.py

@@ -440,7 +440,7 @@ class SkillScores:
         """Calculate CASE IV."""
         AI, BI, CI, data, suffix = self.skill_score_pre_calculations(internal_data, observation_name, forecast_name)
         monthly_mean_external = self.create_monthly_mean_from_daily_data(external_data, index=data.index)
-        data = xr.concat([data, monthly_mean_external], dim="type")
+        data = xr.concat([data, monthly_mean_external], dim="type").dropna(dim="index")
         mean, sigma = suffix["mean"], suffix["sigma"]
         mean_external = monthly_mean_external.mean()
         sigma_external = np.sqrt(monthly_mean_external.var())