MonitorAgent instances seem to share either their buffer or the parameter buffer_size

[BjoernLudwigPTB]

I prepared a branch show_buffering_bug to show, how I come to this conclusion. I included a second monitor agent with a different buffer_size into the basic_buffering tutorial. When running this example both monitor agents' outputs are using the shorter of the two buffer_sizes. I am not sure if they actually share their buffer or if they share the buffer_size attribute.

I would expect, that if I instantiate several monitor agents with differing buffer_sizes, that their plots are using their respective values.

@bangxiangyong Could you take a look at that?