Add types to top-level import to simplify porting from h5py

Previously, types like Dataset would have to be imported
through exdir.core.Dataset, which exposes an internal implementation
detail (everything being in exdir.core).

Type-checking by comparing the type to the one imported is common in h5py code,
which means porting also is cumbersome whenever a comparison is performed.
This changes fixes this issues by exposing the types in the root
import.

exdir/__init__.py

@@ -1,7 +1,7 @@
 from . import core
 from . import plugin_interface
 from . import plugins
-from .core import File, validation
+from .core import File, validation, Attribute, Dataset, Group, Raw, Object
 
 # TODO remove versioneer
 from ._version import get_versions