Fix typo causing error when incorrect param entered.

When entering an incorrect N value (i.e. not a power of two),
the code fails because the exception has been declared of a typo.
Merge pull request #21 from tychobrailleur/master

Fix typo causing error when incorrect param entered.
ui: Stop blocking on plt.show()

After clicking a "Compute" button, the plot gets displayed but (in OSX at least)
the compute button is not clickable again until the plot window closed.  This
makes imposible to display multiple plots at once.

Also, the parent window is not clickable while the plot is being displayed,
making impossible to do certain operations such us playing the synthesized
sounds in the model window.

This patch fixes both problems by changing all the "plt.show()" calls to
"plt.show(block=False)"
Merge pull request #22 from 2opremio/master

ui: Stop blocking on plt.show()