[FEATURE] Introduce the magic method __pow__

[VascoSch92]

Introduce the magic method __pow__ for all the subclasses of _Element.

The power of a permutation can be defined just for integer (positive and negative).

To define a negative power of a permutation, we need to be able to compute its inverse. See issue #4 .

Note that a $\sigma^0=\sigma$ for every permutation $\sigma$ in $S_n$ and every $n \in \mathbb{N}$