names for maps

What about something like the following scheme?:

```
kernelMap f : ker f --> source f
cokernelMap f : target f --> cokernel f
imageMap f : image f --> target f
coimageMap f : source f --> coimage f
coverMap M : cover M --> M
superMap M : M --> super M
```

Recall that for a subquotient module M=P/Q of F, super M is F/Q. We also need a name for
the submodule P/0 of F and its map to M, for completeness, but I can't come up with one
right now: it's dual to super, so what about cosuper?

`coverMap` already exists.