Replace deprecated initialValue with defaultValue

@parenti , I'm tagging you directly since you've pointed out that initialValue is to be replaced with defaultValue.

One line I'm not sure about is in source/pipelining.rst with the following:

.. code-block:: c++

        NDARRAY_ELEMENT(expected).key("arrayStack")
                .shape("-1,100,100") // Variable dimension along the slowest axis
                .readOnly().noInitialValue()
                .commit();

Should this noInitialValue() be also replaced with noDefaultValue()?