strange behavior in kernel (identified in PushForward)

Nobody has claimed this yet.

I've discovered this in the `PushForward` package, but I believe the issue is with `kernel`. Notably, sometimes `pushFwd` gives the wrong answer and other times it has an error. This is an issue on both 1.24.05 and 1.22.

For instance, using a multi-graded setup,
```
Macaulay2, version 1.24.05
with packages: ConwayPolynomials, Elimination, IntegralClosure, InverseSystems,
Isomorphism, LLLBases, MinimalPrimes, OnlineLookup,
PrimaryDecomposition, ReesAlgebra, Saturation, TangentCone,
Truncations, Varieties
loadPackage("PushForward", DebuggingMode=>true);

i1 :
i2 : R = QQ[x,y, Degrees=>{{1,0},{0,1}}]/ideal(x^3-2, y^2+y+1);

i3 : psi = map(R, QQ[]);

o3 : RingMap R <-- QQ[]

i4 : pushFwd(psi)
/usr/share/Macaulay2/PushForward.m2:193:10:(3):[4]: error: not implemented yet:$
/usr/share/Macaulay2/PushForward.m2:193:10:(3):[4]: --entering debugger (type h$
/usr/share/Macaulay2/PushForward.m2:193:9-193:16: --source code:
ke:=kernel mp;
```

In a singly graded setup, I get the right answer.

```
Macaulay2, version 1.24.05
with packages: ConwayPolynomials, Elimination, IntegralClosure, InverseSystems,
Isomorphism, LLLBases, MinimalPrimes, OnlineLookup,
PrimaryDecomposition, ReesAlgebra, Saturation, TangentCone,
Truncations, Varieties

i1 : loadPackage("PushForward", DebuggingMode=>true);

i2 : S = QQ[x,y]/ideal(x^3-2, y^2+y+1);

i3 : psi = map(S, QQ[]);

o3 : RingMap S <-- QQ[]

i4 : pushFwd(psi)

6
o4 = ((QQ[]) , | 1 x x2 x2y xy y |,
--------------------------------------------------------------------------
FunctionClosure[/usr/share/Macaulay2/PushForward.m2:101:17-101:31])

o4 : Sequence
```

What's worse though, in the multi-graded setup, sometimes I get no error but instead the wrong answer if the source is QQ (instead of QQ[]). In the singly graded setup I don't have this issue.

```
Macaulay2, version 1.24.05
with packages: ConwayPolynomials, Elimination, IntegralClosure, InverseSystems,
Isomorphism, LLLBases, MinimalPrimes, OnlineLookup,
PrimaryDecomposition, ReesAlgebra, Saturation, TangentCone,
Truncations, Varieties

i1 : loadPackage("PushForward", DebuggingMode=>true);

i2 : R = QQ[x]/ideal(x^3-2);

i3 : S = R[y]/ideal(y^2+y+1);

i4 : U = (flattenRing S)#0;

i5 : pushFwd(map(U, QQ))

o5 = (cokernel | 0 0 0 0 |, | 1 y yx yx2 x x2 |,
| 0 0 0 0 |
| 1 0 0 0 |
| 0 1 0 0 |
| 0 0 1 0 |
| 0 0 0 1 |
--------------------------------------------------------------------------
FunctionClosure[/usr/share/Macaulay2/PushForward.m2:101:17-101:31])

o5 : Sequence

i6 : degrees U

o6 = {{1, 0}, {0, 1}}

o6 : List

i7 : describe U

QQ[y, x]
o7 = --------------------
3 2
(x - 2, y + y + 1)
```

You can see in the above I am getting the wrong module out of `pushFwd`

Reproduce the multigraded examples in the issue with PushForward and inspect PushForward.m2, especially line 193 where `ke := kernel mp` fails and the function around line 101. Compare the QQ[] and QQ cases; done means `pushFwd` returns the correct module rather than an error or the incorrect basis shown.

Written by the indexing model from the issue text.