Issue 18/within range handler

[mikeAdamss]

Start of the WITHIN lookup engine. Draft PR with a few things still to do.

Things to do:

• read and play, take the time to get it first, this is not a trivial thing to understand.
• complete scenarios 3-8 in within.py, they should follow the same patterns already established with scenario 1 and 2 (do create more methods like those on 145 and 155, it's cleaner).
• tests, this will need a lot of tests
• test the FAILURES as well, we need this to be doing sane things if a DE passed in bad arguments, or specifies a range with no valid lookup in it.
• try it on a very large dataset, this may well need some refactoring for performance once it's confirmed as working (I'm not 100% that it'll have bad performance but we spent a lot of time on performance for the other engines, be silly to take a chance now). We don't necessarily need great performance right off the bat (though it'd be nice!) but we'll at least need to know if it needs looking at it in the future.
• general neatening of things up, this is functional but very verbose, try and see if we can tighten up the code a bit anywhere ("no" is fine, but it's worth a pass or two and a few thoughts).

[CharlesRendle]

Ready for review.

All scenarios completed with accompanying tests. Have left the lookup functions in their verbose forms with the aim that this can be improved upon.

Please look into the within engine functions for searching up and down columns as I am not entirely happy with my logic behind them. Also, I imagine there needs to be a wider breadth for the exception test scenarios. Any comments there would be great.