Added the regrid and interpolate section of the userguide.

I've put a build of this up at http://pelson.github.io/iris/html/userguide/interpolation_and_regridding.html

For significant editorial comments, please consider submitting a PR against this branch.
Simply that I wanted to do it to all four plots, not just the three in the loop, and I didn't really want to duplicate the coastlines etc.

I'd be happy enough to change it - I think I would probably keep track of the axes and iterate over a list of them, rather than duplicate the code, but it would only add 2 extra lines doing so.

Your call.