Next plot behavior

[watermarkhu]

Improve next plot button/enter behavior.
Catch closed window error.

[watermarkhu]

Closing the matplotlib window during a plot will cause an error, or the window is reinitiated (on Mac)
Better behavior would be to continue to the simulation without plot.

[watermarkhu]

Improved in #26.
Still gives segmentation error if new simulation is requested after the figure is closed.