Made output like in the IPython Jupyter kernel:

[sebasguts]

• The stdout is flushed before the output is returned
• Only the last result is rendered as output (if not NULL)

[markuspf]

To record my objection that I brought forward in a in-person discussion: While flushing stdout before printing values of result expressions is a good idea, I don't think we should only be printing the result of the last expression in the cell: It's inconsitent with how the GAP REPL behaves now; the Python REPL behaves this way, too.

The user can manually suppress output from statements in a cell by using a double semicolon.

I am not saying I am not willing to change my mind on this, but I stay unconvinced for the time being.

[sebasguts]

I will close this PR for now and remove the commit from my master branch.