LIU-207 expose buffer protocol getter for data drops

[calgray]

Numerous python libraries accept the buffer protocol for reading in-memory data (in some cases with zero copy). This change adds the buffer() getter that is used by nifty gridder for reading .npy format data. e.g.

def drop_to_numpy(drop) -> np.ndarray:
    bio = drop.getIO()
    bio.open(OpenMode.OPEN_READ)
    buf = bio.buffer()
    res = np.load(io.BytesIO(buf))
    bio.close()
    return res

def numpy_to_drop(array: np.ndarray, drop):
    buf = io.BytesIO()
    np.save(buf, array)
    drop.write(buf.getbuffer())

[coveralls]

Coverage decreased (-0.008%) to 78.343% when pulling b80c5b9 on liu-207 into 7b3be1a on master.

[awicenec]

Easy change and seems quite useful. Thanks.