Automatically flush buffer

[rjagerman]

This will add functionality to the BufferedBigMatrix to automatically flush the buffer if it is full.

[rjagerman]

Due to performance reasons I won't do automatic flushing. There are no guarantees when the buffer will be flushed, which makes it hard to optimize. I have instead implemented a more cumbersome but flexible buffered variant that will require manual checking on fullness of buffer and manual flushing.