union domain implementation (#109)

* Union Class
* Implemented Union Tests

---------

Co-authored-by: Dario Coscia <93731561+dario-coscia@users.noreply.github.com>

pina/geometry/__init__.py

@@ -2,9 +2,11 @@ __all__ = [
     'Location',
     'CartesianDomain',
     'EllipsoidDomain',
+    'Union'
 ]
 
 from .location import Location
 from .cartesian import CartesianDomain
 from .ellipsoid import EllipsoidDomain
-from .difference_domain import Difference
+from .difference_domain import Difference
+from .union_domain import Union