Call NDArray close when close text

When cleaning interrupt was merged with NDArray close, NDArray close
call should have been added here so that interrupt is cleaned for Text
too.
Fix NDArray close check for interrupt

This was off by one, which resulted in unnecessary slicing (although the
result is the same).
Allow previous chains to be extended

This commit enables sampling to extend the a chain from a previous call.
This is done for the NumPy array by concatenating more zeros and setting
the draw index to the right position. For SQLite, this involves setting
the draw index to the right position for the given chain.