Update AssignmentCollection.__repr__

`Assignment Collection for y[0,0,0]` is usually not very helpful.

New representation:

```
In [6]:     forward_assignments = pystencils.AssignmentCollection({
   ...:         z[0, 0]: x[0, 0] * sympy.log(x[0, 0] * y[0, 0]),
   ...:     y[0,0] : x[1,1] +1
   ...:             })

In [7]: forward_assignments
Out[7]: AssignmentCollection: z_C, y_C <- f(x_C, x_NE)
```
8 jobs for update-AssignmentCollection.__repr__ in 12 minutes and 35 seconds (queued for 2 seconds)
latest
Name Stage Failure
failed
pycodegen-integration Test
$ git submodule foreach git fetch origin
Entering 'lbmpy'
warning: redirecting to https://i10git.cs.fau.de/pycodegen/lbmpy.git/
Entering 'lbmpy_walberla'
warning: redirecting to https://i10git.cs.fau.de/pycodegen/lbmpy_walberla.git/
Entering 'pygrandchem'
fatal: unable to access 'https://gitlab-ci-token:[MASKED]@i10git.cs.fau.de/pycodegen/pygrandchem/': Failed to connect to i10git.cs.fau.de port 443: Connection refused
Stopping at 'pygrandchem'; script returned non-zero status.
ERROR: Job failed: exit code 1