minimal-conda

- `get_code` will keep the previous behavior of `show_code`
- `get_code_str` will be equivalent to previous `str(show_code(...))`

This adds a function for lazy people who want to write

```python
dh = create_data_handling((20, 30))
dh.add_arrays('x, y(9), z')
```

instead of

```python
dh = create_data_handling((20, 30))
dh.add_array('x')
dh.add_array('y', values_per_cell=9)
dh.add_array('z')
```

because in most cases you want to use more than one array.

otherwise the index gets lost

uses new Field.neighbor_vector

move_constants_before_loop needed to be changed to move already-existing variables up if necesesary and to preserve constness when renaming variables

also add a test for subexpressions