[Carpet] some fixed levels with carpetregrid2

[Luca Baiotti]

> Actually it is possible.  You can define several refined regions.  You 
> would define one refined region containing the non-moving coarse grids 
> which you want.  Additionally, you define moving regions which track 
> e.g. a horizon.  If you ensure that the moving regions are always 
> contained in the non-moving region, then you have the desired effect.  
> However, if the moving regions move too far, the coarse grids would 
> still change.

Good idea, thanks!

Luca.