wittOverring -- Returns the n-th WittOverring of a ring R, or the overring of a witt ring.

Given a polynomial ring R = (ZZ/p) [x_1,..., x_n] over a finite prime field ZZ/p, and an integer $n \geq 1$, it returns and appropriately caches the polynomial ring $(\mathbb{Z}/ p^n)[T_1, \dots , T_n]$, which we call the wittOverring. The reason is that the n-th Witt ring of R is a subring of this wittOverring. Note: given a quotient ring R = S/I, where S is a polynomial ring over a finite prime field, it returns the wittOverring of S. That is to say, the Witt overring does not keep track of relations in the ring. As a consequence, for a prime p wittOverring will return different answers for witt(n, (ZZ/p)[x_1, .. ,x_n]) and witt(n, GF(p)[x_1 .. x_n]); the latter will have one more variable.