Change the writeable flag back to its original state when finished.

Right now, using Cython version of length and set_number_of_points on read-only numpy arrays (as provided by NiBabel) will fail. The reason is memoryview can only be used with writable numpy array (for more details check https://mail.python.org/pipermail/cython-devel/2013-February/003394.html)

This PR makes sure those Cython functions set the writeable flag of numpy arrays to True before processing them and changed it back to its original state once done.