Primitive element for finite fields

Nobody has claimed this yet.

When constructing finite fields using non standard polynomials, the option `PrimitiveElement` seems broken. The default behavior is `FindOne` which should automatically find one primitive element, but it's not working...
```macaulay2
R = ZZ/7[z]/(z^2-z-3);
F = GF(R, PrimitiveElement=>FindOne);
<< F.PrimitiveElement << endl; -- gives z, which is NOT a primitive element
<< #unique apply(48, i->(F.PrimitiveElement)^i) << endl; -- 24
```
Worse, even if a primitive element is specified, it does not respect it.
```macaulay2
R = ZZ/7[z]/(z^2-z-3);
<< #unique apply(48, i->(z+2)^i) << endl; -- 48 so z+2 is a primitive element
F = GF(R, PrimitiveElement=>z+2);
<< F.PrimitiveElement << endl; -- still gives z...
<< #unique apply(48, i->(F.PrimitiveElement)^i) << endl; -- 24
```

Start by locating the GF implementation and its PrimitiveElement handling, then reproduce the two Macaulay2 examples from the issue. Trace both FindOne selection and an explicitly supplied z+2; done means the reported primitive element has 48 distinct powers in the example and an explicit primitive element is respected.

Written by the indexing model from the issue text.