Skip OLS on request

Current status: OLS is always calculated on data. In case only a hyperparameter search is applied, the comparison with the OLS is not that important.

• add parameter to skip OLS calculation
• only estimate OLS if True (should be still default)
• check in postprocessing at which point, MLAir expects to have OLS forecast
• Maybe think about using competitor to indicate usage of OLS instead of parameter proposed above? (Adding OLS to competitor list would trigger OLS calculation)