QuanityWidget and QuantityField don't share signature

Pseudocode 

```python
class QuantityWidget(QuantityFormField)
    def __init__(base_units=, allowed_types=)
```

whereas: 

```python
class QuantityField
    def __init__(base_units=, unit_choices=)
```
semantically `allowed_types` and `unit_choices` are the same thing but are referenced with different names, if I am not mistaken. A streamlined signature would be easier to work with.