Add support for inplace matrix multiplication
----------------------------------------------
It is now possible to perform inplace matrix multiplication
via the ``@=`` operator.

.. code-block:: python

    >>> import numpy as np

    >>> a = np.arange(6).reshape(3, 2)
    >>> print(a)
    [[0 1]
     [2 3]
     [4 5]]

    >>> b = np.ones((2, 2), dtype=int)
    >>> a @= b
    >>> print(a)
    [[1 1]
     [5 5]
     [9 9]]
