H5sparse

New
- `h5sparse.Group` now inherits from `h5py.Group` (7)
- `h5sparse.File` now inherits from `(h5py.File, h5sparse.Group)` (7)
- `h5sparse.Dataset` now inherits from `h5py.Group` (7)
- support `sparse_format` in `create_dataset` (8)
- support `dtype` in `Dataset` (9)

Change
- the input parameter of `h5sparse.Group.__init__()` is changed from `h5py.Group` to `h5py.GroupID` (7)

New
- forward non-sparse data to h5py (8)
- support `in` operation for `h5spars.Group` (8)

Fix
- return `h5sparse.Dataset` in `create_dataset()` (4)

Change
- use `Group.attrs` instead of dataset to represent shape and format (2)
- append to dataset (3)

New
- `create_dataset` from `h5sparse.Dataset` (1)

New
- H5py-like interface for creating sparse matrix dataset.
- H5py-like interface for getting whole sparse matrix or a slice of sparse matrix.