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

tests/test_docstrings.py

@@ -15,6 +15,7 @@ import xdoctest
     "module",
     [
         "lalib",
+        "lalib.domains",
         "lalib.elements",
         "lalib.elements.galois",
         "lalib.fields",