Create function for “if validation_data is not None”

[johannadevos]

Both lightgbm_gridsearch.py and lightgbm_main.py make use of this code:

    if validation_data is not None:
        fit_params['eval_set'] = [(validation_data[predictors].values,
                                   validation_data[target].values)]
        fit_params['early_stopping_rounds'] = early_stopping_rounds
        fit_params['eval_metric'] = 'auc'

This should become a function in lightgbm_functions.py.