Fixes chetan51/nupic.cerebro2#1

In order to keep the slider up to date, we remove the slider and re-instantiate it after each snapshot. This could possibly cause problems in situations in which there is a very long stream, and the snapshot updates are happening very rapidly, and the user tries to move the slider as it is being updated, but this seems to work pretty well in my local tests. The only other way to approach this seemed to be to modify dat.GUI code directly, to allow for a dynamic change to be made to the max value of the slider.