Issue with homology of matrices

When computing the homology of matrices, Macaulay seems to have an issue with composing matrices over a polynomial ring.

```macaulay2
R = QQ[x]
M = matrix{{0,x},{0,0}}
homology(M,M)
```

returns

```
error: expected maps to be composable
```

Intended behavior would be to return QQ, since the kernel of M is R e1 and the image is xR e1.

On the other hand, it is able to return the homology of matrices whose composition is not zero, which isn't meaningful.
```macaulay2
R = QQ
M = matrix{{1,1},{0,0}}
homology(M,M)
```

returns
```
subquotient (| 1 |, | 1 1 |)
| -1 | | 0 0 |

2
ZZ-module, subquotient of ZZ
```

and it should probably just return an error.

Start at the homology(M,M) entry point and reproduce both matrix examples over QQ[x] and QQ. Trace how composability is checked and how nonzero compositions are handled; done means the first example returns the intended homology and the second reports an error.

Written by the indexing model from the issue text.