QuanityWidget and QuantityField don't share signature

[mmarras]

Pseudocode

class QuantityWidget(QuantityFormField)
    def __init__(base_units=, allowed_types=)

whereas:

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.