Add `lalib.domains.Domain`

- this class models domains from linear algebra
  and is needed for the `Vector` class to be created
- `Domain` wraps Python's built-in `frozenset` type
  + they must contain at least one label
  + as a convenience, so-called canonical `Domain`s
    (i.e., with labels `0`, `1`, ...) can be created
    by passing in a positive `int`eger to `Domain()`
- the `Domain.is_canonical` property indicates
  what kind a `Domain` is
- add unit tests for the class
- add extensive documentation for the class