Support read-only numpy array.

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.