Explaining fillGapMatrix(), an upcoming oce function 

I explain how this function works, as a supplement to the documentation. PS: in making this I realized that the words 'span' and 'hop' were both being used. I think I'll change the default to be 1 (from 2 as in the video) and I'll rewrite the docs to explain that this is the max width or height of a gap that can be filled. Maybe I'll call the argument 'gap', actually. I won't bother remaking the video though.