ContinuousSpace comments are behind changes to the class

[sowasser]

MultiGrid was used internally to store agent objects when ContinuousSpace was created, but @JamesArruda updated ContinuousSpace to use a numpy array instead of MultiGrid (in PR #439). Also, relic objects grid_width & grid_height were originally used to construct the MultiGrid. They're no longer used. The comments were properly removed in 10b262a but accidentally snuck back in with merge commit eb443d0.

I plan to submit a pull request shortly to fix the stale comments in the ContinuousSpace class.

[sowasser]

I've created a pull request (#571) with the comment changes.