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 the user everything should work the same.  For now I just added the decorator 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 it... I just thought I would get it the code out in the open before I leave town.

[1720]