[chore] Proloaf warning should only be raised if appropriate

[FrankKr]

openstef/openstef/model/model_creator.py

Lines 16 to 22 in c5ce486

	try:
	from openstef.model.regressors.proloaf import OpenstfProloafRegressor
	except ImportError:
	logger.warning(
	"Proloaf not available, switching to xgboost! See Readme how to install proloaf dependencies"
	)
	OpenstfProloafRegressor = XGBOpenstfRegressor

The warning that the optional proloaf dependency is not installed should only be raised if it is required. Currently it is always logged as a warning, causing for unneeded concern: