Fixed tolerance bug in intersection

As raised by @niallrobinson in the google groups (see https://groups.google.com/forum/#!topic/scitools-iris/jq6mEn32QQo), the current intersection method has some odd behaviour. This turns out to be a tolerance problem which I suspect originates in the wrap_lons helper function. This PR alters the logic in intersection to avoid the issue. I'd love to see @rhattersley and or @bjlittle comment as I believe they know this bit of the code.

*Now targeting v1.7.x*
@esc24 Nice! I really prefer this approach, rather than having to rely on `np.finfo` :scream: