Complete implementation of usm_ndarray writable flag

[ndgrigorian]

The usm_ndarray class has support for a writable flag, but it is unimplemented.

• Python API does not expose a setter
• If set, the writable flag should prevent writing to the usm_ndarray

[oleksandr-pavlyk]

The setter has been implemented in gh-1141. Hence closing this issue as implemented.