Susburrows/add region support amwg set6 new

Adds region functionality for plot set 6 (on top of mods submitted in other pull requests).

This required a small bug fix in uvcdat.py, because strideY was rounded down to zero, which is not allowable.

This was fixed by:
-  self.strideY = int( nlats/nlatvs )
+ self.strideY = max(1, int( nlats/nlatvs ))

Sample image attached:
![settesting_ann_stress_ers-combined](https://cloud.githubusercontent.com/assets/6811403/5498019/fb3fdc90-86cc-11e4-9e7f-742d2559cf14.png)