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.
Woa, not you are not, I am it should be `streamlines[i]` instead of `streamline`.