Add UNION ALL and δ

[RicardoMonteiroSimoes]

UNION ALL is basically Union without the elimination of multiple equal values, and small delta eliminates the multiple equal values

[immutablestate]

RelaX tries to follow the conventions of classical relational algebra, i.e., we are working with sets. That's why every operator through which duplicates could arise implicitly performs duplicate elimination.

The δ operator only makes sense on a version of relational algebra which operates on multisets, which is not what RelaX is doing. Consequently, we will not add it.