Calc number of samples only if needed by model

• Calculate number of samples only if needed (check with model build)
• replace len calculation of get_number_of_samples with __len__() method

added To Do refactoring labels

assigned to @leufen1

added Doing label and removed To Do label

created branch lukas_issue421_feat_calc-number-of-samples-only-if-needed-by-model to address this issue

mentioned in merge request !476 (merged)

mentioned in commit 53966bcc

added merged in Develop label and removed Doing label

marked the checklist item Calculate number of samples only if needed (check with model build) as completed

marked the checklist item replace len calculation of get_number_of_samples with __len__() method as completed

closed with merge request !499 (merged)

mentioned in commit c520802e