Implemented aux array copying in 2d amrclaw

This PR is needed for latest geoclaw changes.

The major change is that when regridding, aux arrays can be copied if a grid overlaps with grids at the same level from the previous time step.  In setaux routine, can check if aux(1,i,j) has the special value NEEDS_TO_BE_SET from the amr_module and only set the values aux(:,i,j) in this case.  Otherwise, can assume aux(:,i,j) for this (i,j) cell  has been properly copied and is already set.   Only useful if computing the aux values is very expensive, as in geoclaw. 

Should be backward compatible -- setting all aux values in setaux without checking should still work.

Sphere boundary conditions for the clamshell mapping from 2d rectangle to the sphere have also been re-implemented, missing from 5.0.  Not yet extensively tested.