'Returns the current position of write head.
Example usage:
>>> record = mx.recordio.MXIndexedRecordIO(\'tmp.idx\', \'tmp.rec\', \'w\')
>>> print(record.tell())
0
>>> for i in range(5):
...     record.write_idx(i, \'record_%d\'%i)
...     print(record.tell())
16
32
48
64
80'

def tell(self):

    assert self.writable
    pos = ctypes.c_size_t()
    check_call(_LIB.MXRecordIOWriterTell(self.handle, ctypes.byref(pos)))
    return pos.value
