Advection clean

This PR will eventually add the ability to:
Determine advection for grids where nx=ny (done)
shift a grid a number of pixels and trim to a sub domain (done)
create a list of grids that using advective interpolation
aggregate grids using these lists to affect a smooth aggregation avoiding the "Chain of Pearls" effect. 


Initial PR for this feature addition. I am checking to see if my unit tests work with Travis so please do not merge as yet.. I still have a few more features I want to add.