automatic re-evaluation of pods/cells

[forrestbao]

A pain of Jupyter is that after the code of a cell is changed, the outputs of all dependent cells remain the same, and a user needs to rerun all dependent cells to see the effect of the change.

We shall allow users to control whether the re-evaluation should happen automatically.

On a related note, Wolfram Mathematica has a similar concept called Delayed Value, which re-evaluate a variable using the current values of variables in its defining expression.

[forrestbao]

We can further let the arrow visualization to show auto-re-evaluation dependency.