feat(memh5): add '.copy' method to MemDiskGroup

Closes #190

caput/memh5.py

@@ -368,8 +368,7 @@ def require_group(self, name):
 
 
 class MemGroup(_BaseGroup):
-    """
-    In memory implementation of the :class:`h5py.Group`.
+    """In memory implementation of the :class:`h5py.Group`.
 
     This class doubles as the memory implementation of :class:`h5py.File`,
     object, since the distinction between a file and a group for in-memory data
@@ -2007,6 +2006,30 @@ def flush(self):
         if self.ondisk:
             self._data.flush()
 
+    def copy(self, shared: list = []) -> MemDiskGroup:
+        """Return a deep copy of this class or subclass.
+
+        Parameters
+        ----------
+        shared
+            dataset names to share (i.e. don't deep copy)
+
+        Returns
+        -------
+        copy
+            Copy of this group. Datasets in shared are shallow copies,
+            everything else is a deep copy
+        """
+        cls = self.__class__.__new__(self.__class__)
+        MemDiskGroup.__init__(cls, distributed=self.distributed, comm=self.comm)
+        deep_group_copy(self._data, cls._data, deep_copy_dsets=True, shared=shared)
+
+        return cls
+
+    def __deepcopy__(self, memo, /) -> MemDiskGroup:
+        """Called when copy.deepcopy is called on this class"""
+        return self.copy()
+
     def save(
         self,
         filename,