Implement an option to load data from different files/`DataFrames`

The idea is, to allow for timeseries with different lengths, without having to join them. Needs to implemented in:

• SaQC.__init__
• __main__

changed milestone to %2.0

allow double columns append number aka myVar and myVar_1

this comes later, see here !357 (comment 162994)

assigned to @palmb

mentioned in merge request !357 (merged)

marked the checklist item SaQC.__init__ as completed

marked the checklist item __main__ as completed

mentioned in commit 28eaf64d

closed with merge request !357 (merged)