robust apriori estimate for short timeseries

When time series are shorter than 1 year, there are issue with calculcating climate stats resulting in NaN information.

• add dropna along time axis in filter.py:create_monthly_mean
• also check filter.py:create_seasonal_hourly_mean if this behaviour happens there

added enhancement forthcoming refactoring labels

assigned to @leufen1

created branch lukas_issue451_feat_robust-apriori-estimate-for-short-timeseries to address this issue

mentioned in merge request !513 (merged)

changed the description

added Doing label and removed forthcoming label

marked the checklist item add dropna along time axis in filter.py:create_monthly_mean as completed

marked the checklist item also check filter.py:create_seasonal_hourly_mean if this behaviour happens there as completed

added .done label and removed Doing label

mentioned in commit 9b22c484

added merged in Develop label and removed .done label

closed

removed merged in Develop label