Add PowerSemigroup

[William-Kenyon]

Fixes #1092, adding PowerSemigroup(S) for a finite semigroup S. The power semigroup of S is the set of non-empty subsets of S, equipped with the operation $XY = \{xy : x \in X, y \in Y\}$.

Some literature includes the empty set in the power semigroup, so it might be useful to add an optional argument for this. I also added a warning if the resulting semigroup is likely to exhaust your memory, given how quickly it grows in relation to the size of S.

My version differs from the suggested code in the issue:

• Each $xY = \{xy : y \in Y\}$ is computed beforehand, so $XY$ is found with $x_1Y \cup x_2Y \cup \cdots $, avoiding further multiplication in S
• Subsets are generated in binary-code order, which makes them easy to inded, avoiding calls to Position()