implement inq1720 using decorator

Hi @jon4than, Rick, Becky, Bob,

Here is what I came up with yesterday for a new way of approaching inquiry [1720](http://mcidas.ssec.wisc.edu/inquiry-v/index.php?inquiry=1720).

I implemented a decorator called "keepMetadata" which you can add to any IDV formula where metadata gets lost in the return value of the formula.  For now I just added these to the IDV formulas "sub" and "makeVector" as a proof of concept.  I think this is nicer than going into the depths of GridMath and GridUtil to try to accomplish the same thing!

Feel free to take this or leave... I just thought I would get it out in the open before I leave town.

[1720]