kernel of ringmap should remember its Groebner basis

The following example shows that kernel is not remembering its Groebner basis. But I think that it can do so.

```
S = ZZ/101[s,t]
R = ZZ/101[a..d]
gbTrace=3
phi = map(S,R,{s^4, s^3*t, s*t^3, t^4})
I = ker phi
gens gb I
```

The method used is to make a ring with both sets of variables, then compute a Groebner basis, then select the elements that are in the subring. These form a Groebner basis, but we should use `forceGB` to remember this fact.

Related: #527

Start by running the supplied ZZ/101 example and tracing the kernel of the ring map through the Groebner-basis computation. Investigate how forceGB is used for computed bases; done means the resulting kernel retains its Groebner basis so gens gb I does not recompute it.

Written by the indexing model from the issue text.