Generate prediction problems without Entity_ID

[PatrikDurdevic]

Add functionality to be able to generate problems in datasets without specifying entity_id. For example, in the Instacart dataset, some of the prediction problems could be:

Predict the number of <order_id> in the next 2w

operations = [AllFilterOp(None), CountAggregationOp(None)]

or

Predict the number of <order_id> where average <product_price> is greater than 100 in the next 2w

operations = [GreaterFilterOp("product_price"), AvgAggregationOp("product_price"), CountAggregationOp(None)]
GreaterFilterOp().threshold = 100