ORM: Add the Entity.get_collection classmethod

This is an alternative for the `collection` class property. Where the
`collection` property uses the current default storage backend, the
`get_collection` allows to specify another specific backend.

The original design intended to support this use-case by allowing a
`Collection` instance to be "called" with a specific backend:

    Entity.collection(backend)

The `.collection` returns a `Collection` instance which is then called
with passing the `backend`, which would return a new `Collection` for
the same entity type, but with the other backend. However, this doesn't
always work, because the `collection` property will load the backend
from the default profile, which will except if not loaded. Although this
scenario is unlikely for normal usage, application developers may use
AiiDA's API with multiple active backends where no default profile is
defined.

The reason for a new method instead of changing the `collection`
property is that that would be backwards incompatible.

aiida/orm/entities.py

@@ -185,6 +185,18 @@ def collection(cls) -> CollectionType:  # pylint: disable=no-self-argument
         """
         return cls._CLS_COLLECTION.get_cached(cls, get_manager().get_profile_storage())
 
+    @classmethod
+    def get_collection(cls, backend: 'StorageBackend'):
+        """Get a collection for objects of this type for a given backend.
+
+        .. note:: Use the ``collection`` class property instead if the currently loaded backend or backend of the
+            default profile should be used.
+
+        :param backend: The backend of the collection to use.
+        :return: A collection object that can be used to access entities of this type.
+        """
+        return cls._CLS_COLLECTION.get_cached(cls, backend)
+
     @classmethod
     def get(cls, **kwargs):
         """Get an entity of the collection matching the given filters.

tests/orm/test_entities.py

@@ -19,6 +19,12 @@
 class TestBackendEntitiesAndCollections:
     """Test backend entities and their collections"""
 
+    def test_get_collection(self, backend):
+        """Test :meth:`aiida.orm.entities.Entity.get_collection`."""
+        user_collection = orm.User.get_collection(backend)
+        assert user_collection is orm.User.collection
+        assert user_collection.backend is backend
+
     def test_collections_cache(self):
         """Make sure that we're not recreating collections each time .collection is called"""
         # Check directly