Fix coeff for indexed

IndexedBase and Indexed are currently not commutative by default in SymPy. This is fine for tensors
but since these two classes are supposed to be lowest common denominator between different use cases I argue that we should make them commutative by default  in order not to confuse users who do not know about tensor algebra. (I was surprised when my code snippet below did not work - the example came up when doing finite differences)

```python
N = Symbol('N', integer=True)
len_y = N
i = Idx('i', len_y-1)
y = IndexedBase('y', shape=(len_y,))
y.is_commutative = True
a = (1/y[i+1]*y[i]).coeff(y[i])
b = (y[i]/y[i+1]).coeff(y[i])
assert a == b
```

These commits changes the default of IndexedBase and Indexed to be commutative and hence the above example works (added as a test). I really think this is what we want as a default as it makes Indexed instances match Symbol instances more closely.