Provides trait types that support unit groups

It would be convenient if we could define TraitTypes that validate units based on the a quantity

e.g.

``` python
from scimath.units.length import Length, feet, meters
from scimath.units.mass import kg

class MyClass(HasTraits):

unit = Length(feet)

my_class = MyClass()

# This is valid
my_class.unit = meters

# This is invalid
my_class.unit = kg
```

The trait might be as simple as an Enum with all the available unit objects.